Decorating the kid’s bedroom can be a lot of fun and if your child is not too over-opinionated can be a great project to take on together. First set parameters with your child on acceptable colors and designs. Talk through the project, making the child have ownership in the result. Let your child know the budget along with the various costs of furniture, flooring and paint. Let the decorating be a learning experience for your child on operating within a budget. Visit bed stores for things like a twin bed frame and discuss the costs of new furniture by clicking through the different styles and colors along with having a wide variety of styles to look at and develop ideas.
If your child is extremely opinionated, discuss the need of working together on the project so both parties are happy with the final outcome. Decide if your child is old enough to contribute to some of the costs of redecorating or earning something special for the room by doing extra chores.
While discussing the money aspect, also discuss the practical side of the choices of the furniture along with the safety aspects. Also discuss the longevity of the project and how soon the child might outgrow the décor. You want things fun but also practical.