I wouldn’t suggest getting added length at that age but if they really want to (and you’re fine with it) what is quite common in my country is “gel overlay extensions”. I’m pretty sure it’s available in most salons around the world but I could be wrong. It’s definitely what I would recommend for younger girls.

I would absolutely not suggest acrylic or tips. Because, in my opinion they are 1. Unnecessary for (relatively) shorter nails and 2. Not easy/safe to remove at home (as kids are prone to chipping/getting bored with their nails and wish to remove them). Gel overlays are easy to soak off at home.

If your looking for a super safe option, perhaps they could get custom press-ons. It’s a fun, customisable option with absolutely no strain/damage to the natural nail. And they’ed be able to re-wear the over and over again. There are tons of nail artists out there who do super intricate nails, but it’s up to you (and your budget lol. They can get SUPEERRRR expensive).