If you feel very strongly about it, add it back and leave a comment explaining why you've done so, perhaps @mentioning the person who edited it out (so they can respond).
This goes for any sort of edit that you disagree with (whether you're the editor or the, urm, editee). Reverting is optional but you shouldn't do nothing if you think it is the incorrect thing to do.
Not every incident needs to go to meta. I respect that you're asking for the conduct here and not about the removal of the children tag (though I'll say something on that in a minute).
The best course of action is always communication.
Have a word with them in comments. Talk with them in the chat rooms. Find their email in their profile and have a chat. Just remember you're not going to get anywhere unless you're polite, explain your reasoning and also listen to what they've got to say. Ignore those basic concepts and you're going to start an edit war.
If you can't come to a consensus, that's when it's time to go to meta. Be specific. People need to know the post in question and why you're opening the thread. All the moderators get a notification so even if nobody else replies, at least one of us should be able to donate $0.02 to the topic.
About children: I don't see anything wrong with the idea of having a child/infant/young-person oriented tag but I'm not wild about the name "children"... I just don't think I'd neccessarily think to add that tag if I were writing that sort of a post...
But I don't know if there's anything better. Thesaurus.com offers the following synonyms:
adolescent, anklebiter, babe, baby, bairn, bambino, brat, cherub, chick, cub, descendant, dickens, imp, infant, innocent, issue, juvenile, kid, kiddie, lamb, little angel, little darling, little doll, little one, minor, mite, moppet, neonate, nestling, newborn, nipper, nursling, offspring, preteen, progeny, pubescent, shaver, small fry, sprout, squirt, stripling, suckling, tadpole, teen, teenager, teenybopper, toddler, tot, tyke, urchin, whippersnapper, young one, youngster, youth
I find myself wishing we had more use for an anklebiter tag.
No, I think children or child is probably on the right track.