How do I get my concerns across with a stubborn DIL

I’d be more concerned about the formula than the bed situation tbh. However, it’s none of your business really is it?

People get funny ideas about stuff, don't they?
I don't think there's anything you can do about this. Sleeping on the floor isn't too terrible, as long as anything that might harm your GS is removed.
But I do agree about the bottle. Does the GS really need a night feed at 6 months?

I'd advise your son that you don't want to know about the intricacies of raising baby and just sweet photos and nice news.

If plugs are covered and it’s a clean floor I can’t see it’s a problem at 6 months boisterous babies can climb out of cots anyway I remember one of my grandaughters was always climbing out when she was really little ( mind you she was a great gymnast in her school years )

Whilst I respect your opinion, I beg to differ on it not being my business, especially since I’m being included on every step of this process. If I didn’t know, I’d not have anything to say, but since it’s being rammed down my throat, I’d say I have an opinion

It's hard to believe cot mattresses are any more toxic than the single mattress that she's presumably going to use.
But I think you're overthinking this, if the bedroom's fully baby-proofed & they are vigilant, it doesn't sound particularly dangerous to me.

If she is worried about toxins in the mattress, why can't she put a cover over it that would separate the child from its innards. There must be a suitable "waterproof" available, wouldn't you think? Have mattresses changed drastically since her girls slept on them without harm? I can't fathom her concern.

I was lucky when I had my two children. Although our rented flat in an old converted house was cold and inclined to be damp, it was large and so were the rooms. The girls, born 5 years apart, stayed in a cot in our room until they started to climb out of it then, for safety, they got a bed in the hitherto spare bedroom. A floor bed there would definitely not have been a good idea as the only form of heating in the North facing room was a little coal fire. There wasn't even a power socket, let alone central heating. Personally, if such obvious hazards are absent, I can't see the danger in a floor based bed. Just keep the door shut (and get a baby alarm).

I say we were lucky - because I would have been unhappy putting such a young child in a separate nursery room, whether in a cot or a floor-bed. I do realise modern houses often have tiny rooms and offer no reasonable alternative.

I do hope the young lady isn't going to smother instead of mother her much wanted son. My Dad had difficulty keeping my Mum's maternal instincts under control when she eventually produced a son, 13 years after I came along and several years after she was told there could be no more children!

Oh, baby mattresses!
My oldest DD had a cot mattress that was just like an ordinary bed mattress but with a waterproof cover.
Then it was de rigueur to have foam mattresses with holes at the head end. Cot bumpers were 'in' then 'out'. Tummy sleeping, side sleeping, back sleeping as the instructions from Health Visitors changed!
DC1 could climb out of the cot at an early age so I left the side down, with a stairgate in place.
Probably it's better that he can climb out of a low-level bed than tries to climb over the side of a cot and falls.

Fashions change but babies don't.

Somehow my DC

I think they know what they are doing as this is their third chid and as you say the other two are top notch healthy girls.

They love their son and do not want any harm to come to him.

I appreciate everyone’s input on this matter. It’s clear that I am looking at it from the worst case scenario aspect.

It’s their choice to make and I’ll just butt out and what will be, will be.

My two granddaughters both slept in cots.. the eldest was transitioned from cot to a double bed at just over 12 months old but my second granddaughter stayed in her cot a bit longer.

I just have to point out that once the basement has been developed, my son and DiL will sleep in the basement bedroom with the 3 children on the main level. At the very least, I’ll just ask that they invest in a baby monitor for my grandson.

Thank you to all who have responded and made me see things from a different perspective.

"I’ve just voiced my concerns to my son and told him his family here in the UK think it’s utterly insane" .... well that will make you popular!!

Honestly, they are experienced parents, and no doubt they will have some sort of baby monitor on the go and other safeguards.

Please try not to worry.

I would be more worried about the concentration on the Golden Boy and how his big sisters might feel.

There is a strong case to be made out for zipping the lip if you wish to continue good relations with son and DIL. I am not surprised you have not heard back from your son!!

Different people have different fears.
I was babysitting a little boy when I was a teenager. He was so active in his highchair it tumbled over and he crashed onto the floor hurting himself badly. Luckily this didn't happen on my watch but I vowed never to put a child of mine in a highchair.

On the other hand, the popular advice in the 1970s was baby on tummy, and looking back I wonder mine didn't suffocate. By the timeDD came along in the 1980s, it had changed to back sleeping.
How did we survive or our DC.
I have 7 DGC but I don't involve myself in their care. I'm just glad they are happy and healthy.

I really wouldn't worry if I were you. Your DC seem to be very good, caring parents.

Cot mattresses (and child sized ones) can release toxic fumes.

It's known as "off-gassing."
The University of Toronto, has done lots of tesearch into the chemicals found in infant and children's mattresses.

I'll see if I can find the research.

Cots are safe until they climb out of them. And you don’t know at what age that will be. I’m sure I found my son on top of his nursery wardrobe one morning. I did see one of those house building/renovating programmes in which the couple featured had children that were allergic to just about everything so all the furniture, sofas etc were old because they didn’t have toxic chemicals in them.

"Are Sleeping Children Exposed to Plasticizers, Flame Retardants, and UV-Filters from Their Mattresses?"

pubs.acs.org/doi/10.1021/acs.est.5c03560

"Young Children’s Exposure to Chemicals of Concern in Their Sleeping Environment: An In-Home Study"

pubs.acs.org/doi/10.1021/acs.estlett.5c00051

I think these 2 are the ones I read about.

I would be very concerned about the bottle. Not so much the bed.

I don't see the problem?
At some point they all climb out of a cot. If he's already at floor level that's one less hazard.
As long as the door is closed and he can't get to stairs he can safely potter about his room.

OP
Have you tried asking on MN ?

All three of mine were put to sleep on their tummies as we were told to do I knew nothing about poisonous fumes or anything else Then as older children they slept with me in one double bed for a while, (Mum and eldest at top end brother and youngest other end ) better than being homeless They ve all grown into brilliant successful people and good parents to my seven (now grown and flown) grandkids

I remember putting mine face down on a visivent mattress and was horrified when my mother in law laid my first baby on her back.

We had a 3ft 6 inch wide mattress on the floor for DS in his small safety proofed room with a strong baby gate on the door and our door open.

He had a longish period of night waking and one of us could get beside him in the mattress for a cuddle back to sleep then slip back. Worked really well.

(that would be on the mattress from about 11 months when the cot started getting a right bashing...)

No worries here. My mother put us all in a bureau drawer or woven straw basket.

Her baby, not yours.

Shes not stubborn.