I think kids (in general) are a lot more flexible with their imaginations than you give them credit for. Kids of all shapes, sizes, races, and genders dress up as Disney characters. When we go to Halloween events we see kids dressed up as characters who, in the movie, were portrayed to look different than the child wearing the costume. And the kids are fine with that. I've seen other little kids point to an "Elsa" and say "There's another Elsa!!" (Because there are always about 90 of them. ) and they don't say "Oh, that girl can't be Elsa. She doesn't look like her." Nope. You put on the Elsa dress, and you're Elsa. You don't even need the wig. Lots of kids ditch the wig eventually, anyway.
Same with Santa Claus or the Easter Bunny....every decoration, mall version, movie version looks different, and yet kids readily accept that all of those different Santas & Easter Bunnies as completely fine. Why? Partly because parents tell them things like: "It's just a story about Santa, or the Easter Bunny, or the Tooth Fairy, and that's how the person telling this story or drawing this story imagined Santa (etc) to look." Same with Ariel or any other character.
Take a look at how JK Rowling imagined mermaids! Now there's a version of Ariel that might freak a kid out.
But that's just it...we all get to imagine a mermaid however we want and we can change how we imagine them from one day to another, or multiple times within a day. If you ask a child to draw a mermaid they will. Then if you ask them to draw a mermaid again, immediately, but ask them to imagine it differently, they can do that. Kids, in general, are good at imagination.
If a parent simply says "There's a new Little Mermaid movie and this time they used real people instead of drawings.", the kids are already expecting a change. Take one second longer to explain that they decided that Ariel would look different than she did in the last movie, and most kids will accept that just fine.