Unless they can afford to buy the things and still then they must be approved by us.
The way I see it, it's our home and as adults we know what will look good and will grow with our children rather than decking it out in barbie crap just so she can change her mind in a month. We don't give them boring rooms. I always make sure the rooms look wonderful and we spend quite a bit of money making them perfect. So far we haven't had anyone complain and the kids are 15, 10, and 5. We ran in to some head butts with dd15 once but we came to a compromise and she realized my way would work out better.
They can decorate it with home made things like crafts or pictures but as far as picking out their own bedding, wall colors, other decor, and furniture? That's up to me and dh.
I'm sure I'll get bashed but oh well. That's how it is in this family. I've had many people praise and bash me for it irl.