Joking aside, I do genuinely think a shower curtain with a cut down the middle is a good idea.
You can get loads of different designs that will fit with whatever they like. It's cheap, easily replaceable and safe*. Plus it's moderately weather proof.
If you have the will the best thing would be to hem and sew some Velcro so it can be stuck to itself in the center. Or if you cba fold it so it overlaps.
*Assuming if you're putting them in a treehouse they're old enough not to suffocate on it.
Joking aside, I do genuinely think a shower curtain with a cut down the middle is a good idea.
You can get loads of different designs that will fit with whatever they like. It's cheap, easily replaceable and safe*. Plus it's moderately weather proof.
If you have the will the best thing would be to hem and sew some Velcro so it can be stuck to itself in the center. Or if you cba fold it so it overlaps.
*Assuming if you're putting them in a treehouse they're old enough not to suffocate on it.