First time this has happened to me, my LO is only 15 months, walking well, had gone about 30 yards from me in the park to watch some primary school children running around. She sat down for a rest, I went over and a little boy, about 3 came over to her and very deliberately trod on her ankle.
So I did my best evil witch impression, and hissed "Dont tread on my child little boy" with full narrowed eyes and glare. (Overprotective mother of PFB).
To which his mother stood nearby said "oh I dont think he would do that".
To which I said, "well he did". And much to my delight a millisecond after I had said it, so did one of the little girls. (always nice to be backed up even by a 6 year old). And I sheparded off my tot in a cheerful manner.
Now, I suspect what i should do is criticise the behaviour not the child? So said, "treading on other children isnt nice little boy". (?) But what do you say to the mother who claims that little boy wouldn't do what you have just seen him do?