Board size for 7-9 yrs. old / 30 kgs.?

Hi, my two sons learned to waterski last summer and they want to try wakeboarding. What size board do I need? Any recommendations? I live in Belgium and i'm going to Las Vegas in early June, are there any shops near LV? Here we have limited choice and the prices are double, so it would be nice to pick up a board there.
Thanks.

Liquid Force Trip 128 works great for teaching kids. This is my daughter at age 6.