I'll just reiterate what others have said:

1) Don't ever co-sign.
2) Great idea! Nothing wrong with working for a living.
3) I wouldn't give money with strings attached.

I am not a big fan of giving people presents with strings attached. If you want to pay for a car, insurance, their education or anything else then do so. But I think you're asking for trouble if you give them money and ask them to do something with it.... Giving someone money with strings attached will only cause trouble, imho.

Now if YOU want to open accounts in their names (not sure if you can do this with the 19 year old), that to me is different. That's buying them stock as a present. Or buying them a CD as an investment. But I wouldn't just hand them the cash and have them go invest it. I'd have them do it WITH you maybe, but I don't think the money should ever go directly to them.