You should give your son's family the gift of independence. All this showering of money and "gifts" is counterproductive to that. It just makes them dependent on receiving welfare from the pockets of mom and dad.

If you wish to give, direct the giving to the grandkids. Maybe some small toy, on their birthdays and Christmas, and the bulk of the money into a family RESP for them. If the parents are short of money, they have to learn to deal with it, but you can help with the future of the grandkids, since it reads that the parents won't be able to contribute.