It’s easier to explain to them upgraded tails, harder to explain tails that are downgraded or different from your “main” tail.
I tell children that mermaids tails change depending on different factors- for instance, some mermaids tails change as they grow older. When my Mertailor tail was on it’s last leg and wouldn’t hold paint, I’d actually show them the flaking paint and explain that I was in the process of molting and that I would have a whole new tail in a few months (my Merbellas tail). It was actually really convincing, and kept them from wondering about the patches of missing or flaking paint. I tell them that other factors can play a role too, like if mermaids go from one ocean to another sometimes their tail will change (mermaid often pack different tails when they travel depending on luggage situations), or some mermaids tails even change with the seasons (explaining repainting/touch up/change in paint on tails that people tend to do when summer hits, or if they get a new tail in time for summer).
The point to get across is that mermaid magic is wonderful but can be unpredictable- and it effects every mermaid differently at different times! This way you never box yourself in. After all, different mermaids have different answers for everything, and if you say something is one way and they say it’s another, it can ruin it for the kid who is suddenly questioning everything because they’ve “caught” you in a lie.
For example when I’m asked about mermaids going from tails to legs and vise versa, I tell them that there are different ways to use mermaid magic to get legs- some dry off and they get legs, some use magic potions, some use magic crystals, ect. And when I’m asked if I was born a mermaid I say yes (my back story is that my mother was a mermaid and my father a fairy, so I’m half mer half fae) but that not all mermaids are born mermaids- that sometimes they are turned by mermaid magic. This is a big one I like to leave open since a lot of movies/stories/TV shows- like H2O Just Add Water, which is really popular- can go along that route.
For Faerieworlds, obviously I downgraded from my full silicone tail to a fabric tail, to fit with Una’s protocol for the lagoon. Since I live in WA and the event was in OR I didn’t run into any kids that knew me. However, I did get questions like, “Why don’t you have scales?” to which Merlissa chimed in that my scales were just super small- like sharks, which actually have scales, but they’re so small you can’t see them (they’re called placoid scales for anyone who is curious about that). Also, “Why does your tail feel like fabric?” to which I replied that they couldn’t bring in ocean water for the lagoon, and they had to put chlorine in the freshwater to keep it safe with all the germs, and that the chlorine made my sales all mushy. The chlorine also explains why I can’t breathe underwater in pools or take a nap in the lagoon - because I have to tuck my gills away or the chlorine will burn them.
Honestly though, most of the target audience for pro mermaids (6 and under) are typically so shy or excited that they barely touch your tail before excitedly or jumpily snatching their hand back. I’ve had kids tell their friends after they’ve touched a fabric tail that it felt like all kinds of things- textures like scales, dolphin, shark, slimy- none of which was even close to the texture of fabric ;) If they are young and have an imagination- which is the kids we’re aiming our performances at- they will feel what they want to feel!
A lot of older kids may ask what your tail is made of, mostly because they want a tail too. “Magic of course!” or “Scales and magic, silly!” is a good enough reply that it’s usually enough for them to not ask any further- possibly because they “get it” that you’re just “in character” and don’t want to waste their time and/or ruin it for the small kids. If kids get obnoxious and start calling you “fake” and making a scene, you just need to put on your grown-up voice and say that they need to respect you or leave. I haven’t had to do this myself, but I’ve seen other mermaids do so, and it works well. One such thing I overheard this weekend was, ”How would you feel if I came into your home and started calling you fake and being mean? That wouldn’t make you feel very good would it? So you need to respect me and my space, or leave.”
Monofins in fabric tails are easily explained away as being bones and/or cartilage. Knees can be explained by saying you have bones in your tail just like humans have bones in their legs.
Hope this answer helps you out- good luck with your mermaiding and saving up for your dream tail!