How many times did I say that to my kids? More times than I can count. I hope I can offer some suggestions and tips that will make it easier for parents and kids to get the bedroom clean with minimal friction.
Parents: Make your child's room an atmosphere they will want to keep clean. Sometimes we decorate our kid's rooms as we would want them. Be sure the decor is pleasing to the child, not just to you. Ask them what they would like, and let them make color choices, ( within reason. Our son wanted to paint his room black. I put my foot down on that one.) Allow your children to browse through magazines and look for rooms or features they would like to have.
"A place for everything and everything in it's place" is a valuable reminder of how to keep order. I sometimes come across items when I am cleaning house that just don't seem to have "a place". You can waste a lot of time trying to figure out what to do with that item. Adding it to a designated place where it doesn't belong can throw your order out of whack and can become a slippery slope to chaos.
Children need colorful baskets and bins that are designated for specific items. They may need labels to remind them what goes where. Make it easy for children to keep things in order. Put things at your child's level so they don't have to throw things up high where they can't reach. They need convenient hooks for handing up jackets, caps, mittens, backpacks and they should be within their reach.
Rooms need to be set up with designated areas for specific purposes. If your child plays in his/her room you will need to arrange for areas for art, puzzles and games, clothes, school work and supplies, collections, etc.
Children need different things at different ages. Keep that in mind when planning the room. Make sure furniture is the right size for the age of the child. Give them adequate light for working on homework or puzzles, etc. Organization is more important than cutesy themes, although they can sometimes go together, but keep in mind that you want the kids to learn to clean up after themselves so make it fun and easy for them. Make sure the space or container is large enough and the right shape to hold the things that are supposed to go in it. A round basket is not a good holder for rectangular coloring books.
Re-evaluate the plan periodically. Remember children grow, needs and interests change. Always include your children in the changes and the planning, even in the implementing. Do they need larger furniture now? Are they more interested in reading than in art? Do they need a computer and a place to use it?
Make it a good experience for the child when it is time to give up something that is outgrown. Have children help with selection, repairs and mending of old furniture, toys and clothing. This is the age of too much help your children learn to live comfortably with less. Thin out and get rid of the items that are not repairable or useable any more. Make the sorting purposeful. "You have outgrown these things but there are other children, less fortunate,who can use them. Let's fix them up and donate them to others."
Decorating a child's room doesn't have to be expensive. Often useful pieces of furniture that are good for display or storage can be found at yard sales or second hand stores, or even swap meets. Scratches, and damage can be mended or repaired, and colorful paint will make it look like new. Creativity and imaginations are key ingredients. Always keep the size of the child, the size of the room, and the storage needs in mind when you shop and plan.