Just don't let the children know that you're doing this for them, and preferably opt for a system where they can't get their hands on it until they're at least 21, otherwise they'll rush out at 18 and blue the lot in nightclubs or something equally stupid. Few teenagers are sensible enough at 18 to spend it wisely. You can set up a Trust Fund via Post Office Savings where the money won't be available until age 21 but you'll have to first produce the grandshildrens' birth certificates as identity evidence to set them up.
I have a regular savings account with ING Direct for my grand-daughter. Its still my money, but my daughter (her mother) knows that the money is intended for the child if I pop my clogs before I decide when and how much I am prepared to release. A monthly direct debit is taken from my bank account and transferred directly to an account in my grand-daughter's name where it earns interest. I have other named accounts such as Savings; Heating Oil; Dental plan; Car; holidays etc. etc.