I've a very skinny 7yo and it is really hard to get trousers that will stay up! I've tried adding elastic to jeans and so on, but almost the only pair that will stay up I bought in a charity shop - brand IANA - and as they are age 5 they are getting way too short...anyone able to help? I've tried Boden, M and S, Next and none are elasticated enough, and even with belts (when there are belt loops!) the trousers just hang down...any tips please?