Young mother, how to protect surviving children in a Will

I have been getting to the point where, in thinking of one son who has already had a heart attack, I have been wanting to have other's opinions and ideas concerning will making.
He is in his second marriage. They have no children together. He has one daughter, but I really don't think I want her to get his whole share at her age. She is married and has her own life but her mother (dominates her) would totally waste it. I will spare you the gory details there.

But I would like to give granddaughter something if I should happen to survive my son. Also I would like to give something to his new wife.

Maybe there will be nothing to fight about when the time comes, but right now she would stand to get $200,000 if son's present wife was excluded.

Anyone have an opinion on what would be a good amount to give them? I was thinking some amount, or if the estate was in sad shape, then split it between the other two kids and grandchild, equally, whichever amount was the lowest.

Does this sound like an idea? How many hard feelings would this create?
I have always believed the actual surviving children should get most of an estate, so they can spend it before their children get their hands on it.

Any suggestions at all would be appreciated.