It is the parents' decision - no-one else's.
The OP asked what she should do and the answer is nothing at all - especially if she wants to have a continuing relationship with her son, DIL and GC.
Sometimes it is better to keep quiet.
The child will not be in nappies for ever - that is not going to happen. Many children nowadays are potty-trained later than was the fashion when we were young, or young parents; but they do not finish up taking their GCSEs in nappies - fear not.
What is at stake here is not the child's well-being - he will be fine. It is the OP's future relationship with her DIL, son and GC that is in jeopardy if she interferes.
It matters not at all whether the grandparents, or indeed anyone else, disagree. The decision rests with the parents.
I am sure there are many grans on this site who will tell you that they have learned to zip the lip!