The bad thing about this is probably that children who don't comprehend what big of a deal trans can mean to people. It could end up with kids trolling the system, I can imagine boys going into the girls' locker rooms just for fun.
Having that said, the idea is good. I can only imagine that hardships transpeople must go through gender normativity throughout their lives. I do think it's important to create an environment for children where they are welcome for who they are; whether it be trans and be accepted for what gender they feel they belong to, or liking someone of the same gender.
Going back to my first argument about children joshing around is also a bit problematic. Mostly because, while it's not uncommon for children to joke around, a child can also joke around to test boundaries. They might actually have some transidentity, and joking around about it could be a way for this person to "make a bet with playmoney".
In Sweden, there's a school that attracts a lot of people with anormative gender identities, as well as gays and stuff. They recently got a third locker-room where people who want an alternative to the boys' and girls' locker-room can change. Having that said, a person who identifies as either a boy or girl would probably be allowed to go in to the locker-room they see fit; just saying that they got an alternative.