Couldn't you just consider the gift "inheritance" and be done with it? As of 2016 you don't pay any taxes on inheritance/gifts up to $5.45 million per giver over their lifetime, or $10.9 million for a married couple. Giving them even $100k each when it's time will barely put a dent in your exemption. I suppose you need to see if your state has a gift tax you're not exempt from though.
I'm sure there are more tax efficient vehicles, but if you want it to be usable for anything, a taxable account seems the way to go.