If DIL is at home to care for the child it seems illogical to put him through this as he is obviously not ready. If she had to return to work she would have no choice and the wee chap would have to endure it.
How very difficult for you being estranged from her (and worse that you do not know why) and unable to even chat with her about it. But in the end the decision is hers and your son's.
I am puzzled by some of the attitudes of young parents who seem so concerned that their children should race through milestones as soon as possible. There is no reason why your DGS should be ready for this step; and no earthly reason why he should be put through it.
It sounds as though DIL is under some sort of stress at present and he will be picking up on that for sure. I am sure that in the long run all will be well.
Anxiety over gc going on days out

