Decorate with what they like. Barbie, Wonder Women , sports theme, stars/galaxies , Advengers, Star Wars. Ask for their input.
Hi Paula, Stephanie from Abbotts at home is one of my favorite bloggers. Here are some of her DIY ideas in a round up, hope it helps inspire! https://abbottsathome.com/diy-kids-room-ideas-tutorials/
My grandson has always wanted to be a director of movies. So his Mom decorated his room in framed movie posters, she bought a rug that looked like pixels and he had a directors chair that said "director". Here are pics of my granddaughters attic room here, when she was a teenager. Bridle pieces are cause she rode. I also spelled out her name in seashells cause we live in Maine near the ocean. the "green" room was hers here when she was just out of a crib. Its a tiny space. When it came time for her brother to use it, he had a "car" bed.
Choose a theme. Perhaps you can paint images in the theme on the wall.
I would do some DIY based on the theme/what they like