So I have a character that the readers can customize, but I want to create a flashback of that character when they were a child- with the reader’s customization. But, the younger version of that character will have slightly different attributes (to make it look more like a kid) so how would I do that?

I think that you would just need to do something like this:
I recommend to use this:
@CHARACTER previews feature Color/Shape/Hairstyle
@CHARACTER unpreviews feature Color/Shape/Hairstyle
Found on: https://www.dara-amarie.com/script-commands
Most people only change face shape, hair, and eyes/lips.
(And ofc makes them shorter than the other characters, less revealing clothes…)
I don’t recommend using “changes”.
I would probably make a duplicate and then use the becomes command.
So if your duplicate is called MC KID you’d do something like this:
@MC KID becomes MC
@MC KID changes into (features you want the kid to have)
When you use the becomes command, the duplicate character will become an exact copy of the MC, and you’ll only have to di that one time.
I absolutely love Clarkies family CC scripts for this. It helps customize duplicates, young versions, and if you want parents, grandparents, and siblings who look related without looking the same.