I think that you ds and dil have chosen to have a child when they are not in a position to look after the child.
Their shift patterns and their earnings are not conducive to having a baby.
Whilst it is very nice to have GPS look after the child on occasion, to expect them to cover all working shifts without any expense is either because they are a spoilt cheeky f**king brat or they cant afford the cost themselves.
Either way I would be asking why they bothered having children when they expect everyone else to pay for their choice.
Can I ask how many hours of each day your your dil will be looking after her dc. If you are dropping him in the morning is dgs spending all day at nursery and dil then picks him up.
Is your dil one of those women who looks down her nose at sahms and doesn't appreciate that if she had to actually put her hand in her pocket to pay for childcare it would have a negative effect on the family income.
Your ds also needs to re evaluate his career. He can't just bugger off for 3 months and expect others to pick up the slack for free.
Knew a few army wives and apart from a few p/t jobs most were SAHMs when children arrived because they had been posted to different areas of the country and didn't have the luxury of parents and family near by.
What happens if your ds gets posted to a different area.
The ones I knew a couple requested transfers because of career progression but most were just told they were moving.