The coast and castles route, sustrans route 1 looks reasonably flat, ish,
http://www.cycle-n-sleep.co.uk/elevation/elevcoast.htm, and with some training rides should be do-able for the kids. The pace won't be fast, and 50 miles is only really 5 hours in the saddle, at an average of 10 mph, allowing for stops for lunch ect. should be done reasonably in 6-7 hours total per day. One would hope that as they are in the scouts already, they would have some level of fitness. With Kirstie giving the troop advice on how to go about things they should have a good time.