Since they're young I would go for a Sig Kadet LT-40. It is big, so it's easier to see, handles very well at slow speeds, and floats in for landings. That is what I am flying now and next year I'll get my (now) 7-yr.-old started on it. You'll find adherents to every sort of trainer, so this is just my opinion.

I know it's the upper range of your budget, but $400 per plane will probably be what you end up spending. Buy two radio packages and use the 2nd radio for the 2nd plane's receiver/servos/battery and the transmitter (minus battery, antenna, and crystal) for your trainer box. Having my own trainer box turned out to be essential at my club.

The most important first step is what mstroh3961 said: visit the club and talk to the members for advice, input, and to find out how friendly they are.

-Jeff