I asked them, "What was I supposed to do? Just let the kid wander around lost and not say anything?
Luckily for me the kid was old enough to tell them that "the man found me when I was lost". When the mother was located and brought in to retrieve her child (and it really wasn't her fault - she looked away for 5 seconds and the kid was gone), she grabbed the kid and glared at me like I had tried to snatch her kid. I told her next time I'd just let the kid wander away. She finally thawed a bit, but never did thank me.
I didn't save the kid for in order to be thanked, but it certainly would have been a nice gesture.
Too many people are raising their kids to be terrified of strangers, when statistics show that the vast majority of kidnappers and child molesters are NOT strangers - but people the parents know and trust.
Go figure.
Maybe she was feeling guilty about losing her child, and you just became the convenient scapegoat. Even though a child can wander off easily - you just have to turn your head for a few seconds, like you said - that doesn't mean a parent won't feel guilty about it. Maybe it was easier for her to suspect you rather than blame herself for her child wandering off.