My kids (ages 10 & 12, 60 & 80 lbs) never worry too much about seat fit. They haven't started rolling yet and we are always in protected waters. If you think your boys need a better fit for performance, you might make just interchangeable hip pads or a Wrap around back/hip combination. Either way you could have velcro staps around the cheek plates to hold them in place. I use Banshee adjustable hip pads, I love the fit but changing them isn't as easy as it looks.