I'm just thinking of how often I have had near-misses with running children in supermarkets (when parents are within sight but not doing anything to discourage their offspring from racing around) and I'm a fairly fit person far from my 80s. And that's without wheels involved. The speed of movement is almost irrelevant, as it's the unpredictability of the direction that's at issue.
If children don't know the dangers of playing games (with bikes or without) in an environment where there are people moving around normally (however old they are) then it's really parental responsibility to be aware and stop those games.
I can't see why it would be the child who would be the target of legal action in this case, but that's clearly a difference between US and UK law - here there have been major rows about how responsible older children should be considered in far more unpleasant cases of assault and deaths.