i'd like to say that it is a one off thing...

BUT i say this from the perspective of our own experience where our usually happy go lucky dog (who is lovely with all humans, and is happy with most dogs...bar those that try to 'hump' her lol), once totally lost it at this tween aged boy. it came from nowhere and I was horrified. we were sooo lucky that she was chained to my chair leg as i swear that she wanted to bite him. the poor kid was scared beyond belief and I went home in tears after apologising to the kid and family profusely (they were fine and totally lovely about it, citing that she was *just* being protective of our baby). We were very lucky.
After that i became totally freaked out about taking her out in public (we had always been out and about with her since she was 12 weeks old!). i just lost that bit of trust. because i became so scared to take her out, DH insisted that i call a doggy behavourlist. it was the best move! she was able to guide us through and basically let us (me!) regain my confidence with her by enacting some simple strategies (namely that my dog was never to believe that she was in charge or needing to protect us). funnily enough, it meant that she was not to sit under our chairs at cafes! so now we tie her up behind the table and away from the main footpath and TBH we noticed right away the difference, she stopped being curious about all the passer bys and i noticed that she would even fall asleep and fully relax (which in hindsight i hadnt noticed her do before but never thought much of it ITMS).

I have changed my views with her though, so now when people ask for pats (she is a lab x so gets alot of interest!) i tell them we'd rather not. it sucks as i always had dogs that were very social and chillaxed, but obviously stella aint that lol.

she has never ever gone for anyone since that horrible day 4 years ago, but i took it as a warning. even if the kid had the wrong smell, or had intentions to hurt her (which i dont think he did, he was just being loud and excitable running past her several times) it would still mean that he could of gotten hurt and my dog could of been destroyed.

i dont think you need to get rid of your dog it might mean changing her socialisation situations if it becomes an issue, and it might mean that if you guys do want to take her out to cafes and she gets too narky that a muzzle could be worn.