Kids room ideas!

Hey. I am a proud mom of twins (aged 2) and I am expecting another baby in December. We are quite excited as our family is growing bigger. We have some expansion plans to extend a room which is supposed to be the kid’s room.
We have no idea on how a kids room should be. What kind of doors and windows would be suitable? While researching, I came across vinyl replacement windows and doors. https://www.clerawindows.com/replacement-windows-and-doors-manufacturer-and-installer
Do you think it is a good choice?
We are planning to use wallpapers with cartoon characters. Is that a good idea?
https://www.justkidswallpaper.com/
Or should I paint their walls with some funky colors?
Is there any specific type of beds suitable for kids?
It would be great if someone could answer all my doubts. Thanks in advance!

Great ideas.