The gP could buy one pack of nappies to last them for months and months knowing their GC will be visiting for one day, they could also get a couple of wipes in.
I dont buy what the other posters are saying.
GP don't need to bestow emotional needs onto GC and GC don't need emotional support from their GP, if they get it from parents.
Your also making huge massive presumptions that the child does have SN....maybe mum is lazy as op said she over heard once?
However going with the SN needs or extra support theory, still no need not to fill op and her DH in on whats actually going on..should this be the case.
To explain why they are doing what they are doing, however I think its simply because they probably just like SIL more, get on with her and get more out of the relationship and yes,....if they had to choose which they have...they have chosen SIL.
oP YOUR child will do perfectly well, seeing his GP far less frequently and when their full attention can be bestowed upon him.
He will certainly flourish without them, however, he will be damaged as he grows older and realises he is very much second, soon to be third on GP list.
As I said, there is no need to make anyone feel left out.
I would stand my ground on this, say to DH....I am sorry you still feel the need to please and get validation and praise from your parents, I don't think you will ever get it. The whole situation is toxic and damaging...they can still see DS however, only a few times a .......month/year and only when they can commit their full attention to him...should they wish too.