Dad here. I simply don't offer if it involves their kids, it is far too likely to be misconstrued. Plus, that mom is probably doing all she can to keep it together, and knowing that someone noticed could put her in a worse place mentally. If I'm out with my wife, she has offered out help before, and that is much less likely to be judged. It's just a fact.
(I will offer to do things like take the shopping cart back, but I would do that regardless of whether they have kids with them or not.)
I have sometimes seen little kids wander off while their mom wasn't looking. I do pay attention to where they go, and try to keep them in sight. Sometimes it might have only looked like they were wandering, only to find they were going toward another trusted adult. But other times you can see that moment of panic on the mom's face when they realize what happened. It's enough to be able to point and say "they went that way" and they can go get the kid themselves.
I won't interfere unless the kid is going into a dangerous situation, like a street or into water. Even then, I won't touch the kid unless it's the last possible thing to do to keep them safe, I am more likely to get in their way and ask them where their parents are.
What is the "right" action here then, in your mind? Would you rather a stranger grab the kids hand or pick them up?
Touching someone else's kid without explicit consent from the parent is likely to get you in some trouble unless you can explain to them that you just saved their kid
Yeah, if a parent yells at me for grabbing a kids hand to keep them from running into traffic, I will look them in the face and tell them that I will just let their kid get smeared for 80 yards next time.
"Where are your parents?" the man asked the little girl, as he cut off the direction she was walking
I don't want that to be a narrative that any person can ever tell about me. It could be a totally innocent act, but a sinister framing is easy to assume and impossible to refute.
Well, if the choice is between being perceived as a creep vs watching the kid get hurt, I'll pick the option that doesn't end with an injured kid. But if the kid is just wandering and not in danger, like I said, there's no reason to intervene, for just the reason you listed.