The best way to help kids get over capsizes, adults too, is get them some Laser time. Before long capsizes are the 'brag about' part of sailing. Our club has 4.7 and Radial rigs that we can use to help the smaller ones still get some experience on Lasers without the full rig.
The best floatation solutions I have seen have been two prominate Califorina DS'ers. Bill Fiock has removed the cuddy shelves and put 6" thick foam slabs from the cuddy opening to the forward bulkhead that fit under the notch in the cuddy near the outside edge and are shaped as they reach the waterline area. He has them held in with aluminmum straps. (cant remember exactly how) He has many capsizes experience with this set up and swears by it for easier recovery. It seems to prevent most turtles and with the extra foam floating the boat higher when righting, he has less to bail. He is usually able to recover quickly enough to loose only a few places when racing. His is the old style 60's DS I. Another DS'er, Chuck Wilson, glassed in foam under the cuddy roof. As thick as the cuddy is deep at the forward end and tapered to about 2" thick at the cuddy opening. If you didnt know it was there, you'd never notice it. He thinks adding foam there will prevent turtling also. No news as to how it has worked out, but a sound idea. Both these guys sail in SF Bay and outside at Santa Cruz, so unexpecxted capsizes are a fact of life, even for these Pacific Coast Champions.
Another idea is to fill the area under rail seats and coaming area. Something like a 4" x 4" , or larger, would help the boat float highter. The oldest DS's, three digit sail #'s, have that area sealed by a fore/aft bulkhead running the length of the cockpit, and have the entire stearn area also sealed, like the newer SLI DS's built in the 90's and the current Cape Cod version.
Hope these ideas help. Happy SAFE sailing.
phill