Now my opinion on this now, differs from when I was the 'cavalry'. I certainly would have been less than accommodating if I had been called to a job over children on a swing, but an upset mother saying a strange man is assaulting her child certainly brings a different perspective. A huge amount of variables is at play.

What he did to the child was technically assault (without the battery), but if the mum felt threatened then it is with battery, but I can't hall but wonder what she expected them to do if she didn't want to make a complaint.
No way in hell would I have let that happen with out a fight. But, we only have one version of the story.