What do i do

Can’t you go and lay with them at their house?

Play not lay

Perhaps she just needs to ‘download’ her worries, but doesn’t actually want help?

You’ve posted this on another thread.
There, I suggested you just go to Halfords and buy two seats- they will show you how they operate and then you’re all set up.

But Maw to do as you suggest is to go against the wishes of the children's mother.

No, she hasn’t said NOT to, just prevaricated on the make and expressed doubt on her mothers ability to use them - IMO gaslighting, ( Doesn’t trust. anyone to use them correctly-really? There may be an anxiety issue there)
If OP tells her Halfords say these are the best there’s not a lot D can do.
Alternatively OP could have a sneaky look at what D has in her car.
But be warned OP these do not come cheap these days, perhaps D is trying to save you considerable expense without saying so,

I would just say to your daughter, get the car seats and put them in place, I will give you whatever they cost as I want it be done correctly and that you to feel reassured they are safe. Then leave it up to her.

Since millions of children are strapped safely into these seats everyday, it seems to me that there is an issue behind her behaviour, of which the worry about child safety is merely the outward manifestation.

I think you need to try and consider what that might be and help her resolve that before worrying about car seats. What does her partner have to say on the issue? What is he doing? Is he the cause of the problem?

Perhaps she is not interested in anyone "taking her toddlers"?

Could you say that you will happily take the children to give her a break, could she get the car seats and get them fitted to her satisfaction (even if you have to give her the money.)

You could try a more open ended question, such as "is there anything I can do?"

Perhaps you could take the toddlers out some other way - maybe walk to the nearest park with a palayground to give her a break without the anxiety of driving them. If I visit my grandchildren I sometimes take them out on the bus. They love that as they normally go everywhere by car. We only go a few stops to the park that's a bit too far to walk to but it works for me.
It sounds as if the issue is more than just about car seats but if you suggest alternative you may find that out.

Do you have to take them out in the car? Could you take them to a nearby park or just help her in the home? Maybe she just wants to vent and have you sympathise with her struggles. Maybe it isn't the toddlers who are the problem but the difficulties associated with working. I found that very tough especially when they were ill or childcare fell through. I agree with the people who say, "Just ask what you can do rather than making suggestions." You can only give it a try.

Is there a partner? If so pehaps speak with them about getting car seats, they could even go with you to buy, have fitted. If no partner uou shoukd try irganise same wuth DD. Failing which just leave the situation as is with an offer of being willing to help if there is any other way you can help out.

Can she afford a day or two at a Nursery or can you afford to help out this way.

Could you offer to help another way..do the ironing or cleaning or washing, or even cook and deliver them a dinner once a week?? My mil did that for us ..it was such a help..she just used to drop it off 🥰

I presume she hasn’t got car seats that you could share while she is at work?

I'm sure the car seats will be "wrong".
Please be careful.

She could organise child care, the way so many other working parents have to?

If she actually wants you to take them in your car, why can’t she transfer the seats from her car to yours? If she doesn’t trust you to be driving them, can you mind them occasionally at her house? Does she work from home?

A fact of life for most working parents is having to pay for childcare at least some of the time. If you are retired, fit and healthy, and you think you can manage the two toddlers all day, be wary about committing to too much. Your daughter is earning and needs to take responsibility.

Agree with Doodle…go and play with them at her house,then there’s no need for seats.

don't knock yourself out, I looked after mine for 15 years and now not one of then gives a jot about me, wernt even invited to their weddings

Your daughter sounds a bit like mine was. I wouldn't push the car seat thing.
My DD felt really sad that she had to leave her little boys to go to work. When she wasn't at work, she wanted to spend every moment with them. She would never allow them to stay in our house without her.
So we used to spend quite a lot of time at theirs, looking after the boys and taking over the household chores. Eventually we were allowed to drive the boys. Now they can drive me, hurrah!

I would go with SpringsEternal's and ask her if there's anything you can do to help.

Help out if asked but don't push yourself forward, it's her life if she wants your help she will ask, if she doesnt it's her problem, not yours.