A. In general, an ace wrap or knee sleeve wouldn't be harmful, but it doesn't address the underlying problem that is causing the pain.
Did she sprain it?
Does she have Osgood Schlatters Disease, with a tender bump just below her knee cap?
Or parapatellar knee pain syndrome, with vague pain around her knee cap that is worse after many activities and gets better with rest.
If her knee also develops swelling and occasionally 'gives' or locks, then she could have osteochondritis dissecans as a cause for her knee pain.
See our knee problem symptom guide for more information.
If this is an ongoing problem or is interfering with her activities, an evaluation by your Pediatrician would also be a good idea.