Your kids room is where your child learns how to be independent and territorial. At this age, your child would begin to show independence and privacy. It's very important for young children to know how to keep his own things in order and keep the place clean even though little assistance must be given so he would know that what he does is correct.
When you give your child his own little themed room, it inculcates the feeling of trust in him. Little kids with their own rooms act more mature than those without.
Your role as a parent will be someone who would tell them the basic rules to keeping their room clean. As your kid gets older, they may want different things and their beliefs as well, so it is only but proper to reinvent the designs in his room.
Let your little kid join you as you customize the room, this way the child becomes more aware of the labor needed to have a nice room. It would help a lot if you tell your kid that there is only enough money to pick the items he wants, this way he will also know the value of budgeting. There are websites or softwares online that allows you to redesign a room so you can see how it would look like even if you have not started the renovation yet. By allowing your children to be part of the decorating process, they will feel empowered and really begin to enjoy their own work.