Many people think there's a link between being hyper and eating sugar, but research doesn't back that up. Several studies tested behavior and learning in children given sugar vs. a sugar substitute, and found no difference.
Other studies found that parents who think their children have eaten sugar rate their behavior as more hyper than parents who think their children have had a non-sugar substitute.
This doesn't mean it's a good idea to let your child gorge on sugar, of course, but cutting it from their diet probably won't end the problem.