That sounds exactly like what my daughter, the same age, did. It was a buckle fracture, from bending too far. She didn't even cry at the time, and the next day went to the play park, in and out of a wetsuit to go bodyboarding etc. The only reason we took her is because I had a feeling it looked like she was holding it awkwardly, and couldn't decide if it looked a wee bit swollen.
Docs said that that it is incredibly common in kids her age, their bones are so bendy that they almost bend (buckle) or twist (greenstick). After it happened I chatted to so many parents who have had similar experiences, it happens! It isn't a break, but a fracture. If it was a break, you would see the misshapenness, and she would be in a lot more pain.
Doc explained it as the fractures I described hurt a bit when done, feel like a sprain for a while after (ache) and then fine. Quite often don't need casts, and if they do, sometimes only for 2 weeks. Dd had hers for 4, but was told she could carry on as normal.
I am amazed so many people are suggesting waking a peacefully sleeping 6 yr old to go to A&E on a Saturday night. Go tomorrow morning if it is still swollen, it will be more peaceful then