my daughter in law (not married yet but i think of her as that) has a son (12) and i treated him as a grandson even before they had a child together, i give him money for birthdays and christmas as he saves it to get things like more expensive trainers or something he wants and his mother and my son will pay half the cost, he has his dad but no grandparents alive, i will sometimes see something for the little one who is two but will still give the older one some money when i see him, he is part of my sons family and just as much my grandson as the little one is. the children are part of your sons family so please treat them the same, my mum always got a few extra gifts like toiletries for teenagers and would slip a fiver or tenner in with it along with a selection box, it is not the cost of the gift, it is the fact that you bothered to get them something.