It will work great. I've had a bunch of more or less dedicated beginners boards, but my son (who is still pretty much a beginner and of similar size/age) prefers my Quad 76 and you can in fact see how much easier it is for him to get the small board moving compared to the big ones. Of course, big wide ones make balancing easier, but I reckon that's not a big problem for a kid.
I don't know about the iDO though. But again, with a dedicated kids rig (my son use the Hot Sails Maui Kids superfreak 1.6) uphauling in fact looks amazingly easy for the kids: I'd say 10 times easier than for a grown up to uphaul a 5.0.