personally, I think skim style are easier to teach on than surfstyle.
I used to always answer this question with hyperlite broadcast. large, floats easily and rides forever, but the last few I've taught have had an easier time on my phase 5 daniello pro.

I'd look at the phase5 either prop or razz. both are going to be a tad big for the boys but they will grow into them quickly. both are good boards as your skills progress. the broadcast is just a large floatable sofa that you will outgrow quickly if you stick with wakesurfing..

my .02

oh ya, and get a dedicated wakesurf rope. don't use a wakeboard rope. the chances of getting caught in the handle are not worth it..