I looked at 26" wheeled bikes for my youngest and decided in the end to get a small framed Boardman comp 27'5. Managed to pick it up from halfords on one of there many sales and got a further 10% off through British cycling discount. Cracking bike and I was lucky to get it for under your budget. With 11yr olds they seem to grow very quickly hence me getting the Boardman 16" frame. May be worth waiting for the deals to start coming up.