Reward charts have never worked for me because I can't seem to keep up with them.
We do the FlyLady system (which I'm sure you've heard of on here) and we used to do the House Fairy.
The House Fairy (check the website) comes unexpected to your child's room at night (or when they are not in the room) and checks to see if the room is clean. If the room IS clean, then she leaves a surprise! If it's not clean she leaves fairy dust (confetti).
The website has free videos that tell about the House Fairy so your children can "meet" her. If you sign up you can have access to videos, certificates, etc. but you don't have to to make the House Fairy real.
I would buy $1 surprises from the dollar section at Target or the dollar store. Often I'd buy things my daughter needed, the House Fairy often brought new toothbrushes or hair rubber bands, or new bath soap or bubble bath. She also brought school supplies like packets of pencils or erasers.
Our House Fairy came twice a week, but your House Fairy can come as often as you want. However, the cool thing is the kids never know when the House Fairy is coming so they should keep their room clean at all times just in case. We did a 15 minute cleanup before bed every night, just in case. It's wonderful to make the House Fairy the motivation, and it was easier for me than reward charts. Our House Fairy also left many positive notes in fancy script, and one day I found a box where my daughter had saved all of them!
My daughter is now too old for the House Fairy but she kept her room clean while she was young just in case! Now it's just habit to take 15 minutes to clean up before bed.
It works! And your children are just the right age to enjoy the magic :)