IMHO you were just the thoughtful helpful friendly mom and not the person to be judged or held responsible for the situation. You were there with 5 (young) kids offering to wait to help the two 'lost' girls. Chapeau!
I had a very similar situation and same reaction by dh. As concerning the nature of the situation I was in, ppl might even have more fierce reactions but I'm still convinced I did the thing that was the best to do while everyone else was merely standing by...
Our supermarket has a small indoor play area for age 3-6 next to their own fast food place. The play section is enclosed, there's always one shop staff supervising, and it's explicitly signed that parents are NOT to leave their children unattended, and that the shop nor the staff supervising are responsible for the children. The staff member mainly functions as a helper/superviser. There's an area where the parents can wait and watch the kids play, and communicate whit them when needed.
I've been there often with my kids (yes, often being bored but happy since my kids were having fun) and it happened many times that either parent(s) went of to order meals (from where they could not supervise their children even though it's 'close'), even occasionally some who intended on going shopping while leaving their kid at the playground. On few occasions the staff member was clueless what to do when there was an upset child and obviously no parent waiting at the fence. It happened that she had to, very reluctantly, take the child and look for the parent in the restaurant area, to find them leaving the play area for a short while (which is sure against job regulations).
One time there was a girl who happened to walk out of the room and no-one but the attendant had seen her go out. She ran and brought the girl back and it seemed her parents had 'left her there', staff member relieved she could prevent her from being lost. Later, the girl said she needed to go to the toilet urgently (about 3y old), the attendant couldn't leave and/or help her, no parent of the girl, first was waiting for her parent to show up, then I offered to look for the parent in the dining area while asking the attendant to pls make sure MY kids stayed there and to tell them I'd be right back (as I said come there often). We did not find the parents while standing right outside the play area. I sure wouldnt take her around the whole restaurant. Then the girl told me her parents actually went shopping! Wow! Then she said in tears and worry she really needed to 'go'. I did not know what else to do than to go back in, check with attendant and bystanding parents that I'd take her to the toilet then (right nextdoor) if there was no other way. I had to come back for her shoes which we forgot. Then to the loo. I did not feel super comfortable but my mom's heart could not let this girl wet herself because her mom wouldn't care!! Anyway, it was hardly in time. I brought her back, only got positive looks. Then all of a sudden the mom had showed up and taken her out so quickly that we almost hadn't noticed and the attendant hadn't even had the chance to talk to eher about leaving th child unattended. Us all having the 'poor girl' nod...
My dh also was very upset that I took a girl I didn't know to the toilet. I know if you think of worst case scenario it could be tricky, if you think of it as a story told only. He didnt make me feel good about my descision to help. But I know at that moment I DID the good thing for that girl. She was so lost and lonely and she had experienced there was at least SOMeone who cared. Yes, in a multiple choice question I may think more than twice and say not to do this. But real life has feelings to it, too.
I've also run after a boy on a trike who went around the corner of a street, a street going steep downhill, been running and running my heart out to get him. And luckily there had not been a passing car but few more seconds and he would have smashed a parked car or got killed whereever his adventure would end...
Sometimes you just act and thinking is not the best to do first, sometimes you may have thought better before acting, but the line is not always clear when it's 'urgent' to make a descision.