Jim, check out the Shred Stixx Ringer. It will cover all your riders and will allow you to progress rapidly.
On the small end, you can find a 2011-2012 Shred Stixx Nano (production Nano was 4'2") these can be found on sale for as little as $399 on line, or a TWP grom, is is about 4'0 (I think is is $499?). Both great youth boards.
Skims are great for sinking and getting up, but they ride like skim boards, and can be sort of hetchy sketchy for youths.
For 2013 I updated the Nano profile, and introduced the Skywalker. It is a light weight performance rider board, that comes in at 4'4" in production with a weight range of 50 to 140ish, retail is $499.
On the custom side, I can make a Skywalker as small as 4'0, but would recommend 4'2" for your three youngest sons.
Last edited by Chaos; 04-17-2013 at 10:51 AM.