Re: kids and leg pain
I wouldn't stress it too much.
I've had 3 little kids and when they say they have pain and then they go play, they usually don't have any serious complications from their injury. If he was truly seriously injured, he wouldn't be playing, he'd be laying on the floor crying or at least a little more lethargic. Now you or I if we did that- we'd probably have to go to the doctor's and have everything done! But kids are a little more flexible.
If he did anything to it, he might have pulled a muscle which doesn't feel good. And his other knee might be hurting because he's leaning on it more to favor the other leg. Then the uninjured leg can start to hurt because its compensating for both legs. There's a cream you can buy at most stores called Arnica, this might help the swelling and bruising. (Topical only)
Otherwise, if after a week he's still complaining consistently(and you can determine its not because he's liking the extra attention) he might have actually ripped a tendon and needs to see a doctor, although that would be a more rare occurrence.