That is an excellent question. I have a 1 year old son and have already given some thought to this future problem. What happens when you give your child a large sum of money when they're really still quite young at 18?
My parents had a savings account for the kids and gave it to us when we turned 18. They said we could do with it as we pleased, but should at least consider saving it or spending it on something worthile. It was like $2500. I put most of it aside and ended up buying stock and leveraged products. I messed around with all of kinds of trading strategies back then and learned a lot of valuable lessons, but could've easily lost the entire amount. I got lucky enough to end up with about zero loss after a couple of years. My brothers and sister pissed it away almost immediately, and they're not irresponsible people.
What your kids are going to do depends largely on the way they are, I guess. I'm expecting my kid to be stupid with what we give him, but hopefully learn a lot in the process, enough to not make the same mistakes again, like I did. Because I too am worried what the effect will be of giving my kid a large sum of money, I've decided (and convinced my wife) to keep it low enough that we'll helping him get started, but not pay for his life and possibly mess up the way he looks at money.
Anyway, make sure to post back here in about 4 and 8 years to let us know what happened, that'll leave me some time to make some changes