I have a Formula 161.
To use a formula board for light wind practising rig control (I suppose for your kid) will offer a lot of stability. You can almost dance on this kind of wide board
I don't let my sun of 13 touch my board though. It's much to fragile. A bad crash with the gear can easily cause a crack in the deck.
Watch out with cutdown fins. I've tried once a 50 cm fin in shallow waters. It just doesn't work. The board dances around on the water. You need at least 60 cm or preferrably 70 cm fins. I know it is a pain in shallow waters.