Need help reorganizing kids' rooms!

We're building a new house beside our current trailer, and expecting a third child in November and I'd like to make the transition as easy as possible.

We've got a 14-year old who is somewhat organized. He has LOTS of Legos and has an action figure collection. He's also a gamer.

We have a 7-year old daughter who enjoys barbies, babies, and stuffed animals. Only, she's not very organized, just tosses everything everywhere. Between Dad and Nana, it's just "toss it all in a box" and then it's melt-downs trying to find the missing barbie shoe (which is why I began getting her little boxes for little items like that).

I'd like to start with their rooms, trying to reorganize and filter what we've got (eventually throughout so it's minimal move). I'm just at a loss as to where to start. I used to be somewhat organized, but then was just so overwhelmed with my husband and his family and their "get a toy box and toss it all in there" ways. I started off with little toy shelves and such and then they basically just did away with it and took over.

Thanks for any help and ideas! 😊