Good for you, that's a lovely sentiment. An investment account for them to access when older is a good idea, my plan for my nieces and nephews is to invest in a fund they can access only when they get to university, otherwise at age 30 if they don't go to uni. About the 18 year old, I bet he would love $1,000 upfront but not everyone is financially mature at that age, maybe talk to his mum and dad and see the best for him?
Xx