It takes a long slow approach to bring a child's weight down and establish new, better habits. We didn't really manage it until DD herself was very much on board with it. She was an overweight and permanently hungry 10-year-old and is now a slim 17-year-old.
A few things that helped:
Change4life ran a local programme for 'children who are not as fit as they'd like' (to which you could only be referred if they were overweight). Given a chance to play sports quite hard with other equally crap children, she got a couple of hours of good breathless fun each week. They also got the children to set their own eating and exercise challenge each week (plus one for the parents, which she was merciless about monitoring).
We talked about eating 'the right amount for your age' rather than just less. Our oldest child was very underweight at the time, so this meant we could be consistent.
She was an avid baker, but we steered it gradually into other cooking, not just cake.
She had some physical difficulties in running/jumping which put her off -- if your child has anything similar going on, get to a physio! It was a virtuous circle as straightening a dodgy joint let her exercise more, which took some weight off and lessened the strain on the joint, so she could exercise more...
And finally, she found some sports and outdoor things she loved. We tried all sorts - horse riding, gardening, cycling, rowing, canoeing, hula hooping, geocaching, dogwalking, skipping never again. (I've only just realised that that also meant I've spent many fun hours in her company through her teenage years doing things I'm laughably crap at, and it's probably part of the reason we are close.)