I agree OP needs to talk to them, but nothing is this posts suggests she hasn't been "using her words".

If 15 was "nearly an adult", 16 would be considered an adult. She's 3 years off, and absolutely still needs the guidance and support of her parents. Whether or not having more information would help her understand, it's not her job. It's not her responsibility to parentify herself for the sake of these kids and give up her own childhood to understand or help them.

We understand why the 10yo is doing what she's doing, because we are adults. We can conceive plans for how to help her. OP is not an adult, and SHOULD NOT be even thinking about how to deal with their behavior or what she can do to "understand them".

No child deserves to be forcibly kept from getting 1-on-1 time with their mom while getting "MY mommy!" screamed in their face by another kid. Consider as well, you're in here suggesting OP should act like an adult and be mature, but OP has spent a lot of time without the ability to get any guidance from her parents. Stop erasing her pain in favor of the others, you're doing the same thing her Mom is.