Here's what I witnessed some time ago:
There is a large ground level chess set set at an outdoor mall, with squares about 2x2 feet and pieces 2-3 feet high, about 5 lbs each. This chess set is usually used by small kids, 2-5 year old, who move the large chess pieces, make around themselves, pretend to ride the horses, etc.
One day a much bigger kid, 10 year old or so, apparently elated by the lack of supervision, ran into that chess playground, and started throwing the 5 lbs chess pieces with force across the chess set in random directions, sometimes missing the toddlers by inches. The thrown pieces would sometime bump the other pieces around the toddlers, sometime miss a toddler's head or leg by inches, etc. This lasted for at least 5 min.
And in those 5 min of quite dangerous bombardment not a single adult would attempt to restrain the minor. Some parents would take their toddlers out of the chess board games despite their protests; some would get closer to their kids within the chess board while watching the 10-yo, apparently with intent to intercept a chess piece if it would fly at their kid; some adults would just watch what's happening from the benches. And nobody would interfere with the apparently dangerous youth, not physically, and not even verbally.
I gathered from the conversations nearby that the adults were just afraid to restrain the youth for the fear of being accused later that they were doing something illegal against the minor. The parents were more afraid to be branded abusers of that child than of the physical harm said child would do to their own kids.
Thus the question: how realistic where their fears in the state of California? If an adult physically restrains a child and claims that the child was a danger to others can the adult have legal problems?