I have just found some great things on Etsy. I was looking for prints for my sons' bedroom. I do find it quite hard to find suitable things for boys. I think these tick the right boxes for a 3.5 year old, and a 7 year old.
I especially like Sass and Peril, she has both an etsy shop and a website/ blog. On her flickr site she shows the process of how to do screenprinting, which, I can testify, is a lot of fun to do. You just need a lot of equipment/ darkroom!
Another shop, with some beautiful prints of birds, is Bee Things
If I had some spare cash, I would be investing in some of these lovely prints.