I'm overly cautious about any injuries to knees, joints or feet, as these are critical to skaters, and young skaters in particular have a great deal of vulnerability in these regions. Myself, it would be a) to the paediatrician and b) to the physiotherapist, and, depending on their recommendation, moving forward from there (x-rays, MRI, physiotherapy, ultrasound ... etc etc etc). You could have a small fracture in there, or a soft tissue injury; it's impossible to diagnose yourself, and not treating it properly could lead to a long-term problem.
My guys are healthy, and structurally sound because we've always been aggressive about any injuries in these areas, and take all of the time off that we are directed to do.