Hmmm. I come down on the side of saying that the mother ought to have been more on top of things. Turning your back on a three-year-old for more than three consecutive seconds is asking for trouble. And in this society, nobody else is going to control your child (or more worrisome, protect them in most cases) if you won't.

OTOH, I expect the three-year-old was most likely acting without malice -- just enjoying the pretty colors. So what are monks going to do? Blame a three-year-old for being three, or accept that life is just like that sometimes and move on?