My son and his wife are so messy and unorganised

When I read the title of your post I thought that could be me writing this, until I read further. My DS & DiL are also very messy and unorganised in their own home. It drives crazy when I go to theirs, but that's the way they are. They're very laid back and relaxed. And it's not my place to change what they do, or how they live in their own home.

When they visit us, they do usually dump kiddies stuff all over the place, on the chairs, on the dining table, etc., so it all needs to be cleared before you can sit down or eat a meal. I guess, to a certain extent, you have to expect lots of kiddies stuff when the GCs visit! BUT, they do still discipline their children, and don't allow them to run around with food, etc. And they always make sure that everything is tidied up properly before they leave. They are probably more strict with their children's eating habits than I am to be honest.

Personally, in your position, I wouldn't be allowing them to disrespect your home when they visit. I'd say to them that it's up them what they allow the children to do at home, but when they're in your home they have to respect your rules, and that means eating at the table, or in a high chair (we purchased a very cheap one from IKEA to strap them into!), and no running around with food, etc. otherwise they don't have any, and they certainly don't get ice cream! They'll soon learn to abide your rules!

From what you have posted, it sounds to me that the lovely parents do not have a parenting style but they have a lack of parenting which I view as laziness. Also, it doesn't sound as though they have "skills" in parenting. No parenting Skills. From what you've written, it sounds as though they are raising their children as some sort of animals. I hope I am wrong. However, I would not correct the parents on this but I would play a manners game with the children at my house and be the mean granny, if necessary, who teaches them table manners. For heavens sakes. I am so sorry that you are witnessing this tragedy. It is not right.

I agree with you that their parenting style is chaotic, but their house, their rules. If you go there do not run round attempting to clear up after them because it won't make any difference.
In your house, you have the right to expect that the children wear pants at least, and sit at a table when eating. But if they are allowed to roam free at home it won't be easy for them to comply. These are the sort of children who are arriving at school not toilet trained, and with a complete lack of table manners when eating their school lunch; another job school staff are now expected to undertake.
A difficult situation for you to deal with.

Stop going and judging them

Problem solved

I agree too. You can't do much about their house or parenting skills. It's really up to them.
But when they visit you then you do have the right to expect better. Your house, your choice.

The thing is that you are attacking their parenting skills . Their ways are clearly different from yours, but that’s ok in their own home.

In your house it’s different, and you’d be within your rights to ask them to sit still when eating. Unless they are babies they should be able to adapt to doing things differently at Granny’s.

If I were you I’d stop commenting on what they feed their children, and stop changing them all the time - what’s the point if they are going to get dirty again soon afterwards? Lay down rules about eating at your house but leave them to do the parenting. Could you put a plastic sheet on the floor and ask the children to sit on that picnic style, or only have food when on the mat? That would contain it, and be easy to clear up when they go home.

rafichagran I agree.

Not unreasonable to ask for nappies and food in the kitchen when they come to yours. If you go to theirs, put up with it or dont go.

I would not go to their home if it affects you so much. My take on this is, its OK for them to parent how they like in their own home, but when they are at yours they should not be running around your home with no nappy and food in in their hand.
They are being disrespectful allowing the children to be unruly and it is clearly affecting you.

If you can't bear to be around why are you going?