Etsy has become such a go-to source for affordable artwork that I sometimes forget it's not the only source. I recently discovered Society6 and went on a bookmarking binge. Above are some of my favorites and should give you an idea of the wide selection.
Society6 lets artists submit their work and sell it on different items; paper prints, of course, and also artwork on stretched canvas, shirts and skins for laptops, phones, etc. Not everything is suitable for kids, but I saw so many things I could imagine on a child's walls. Below are the links to some favs, but you'll probably want to dive in for yourself at Society6.
1. Take the Scenic Route print by Alli Coate
2. It Would Have Been Cooler as a Van by Brandon Ortwein
3. Classical Notation by John Tibbott
4. Plenty of Fish in the C by Jay Fleck
5. Night Owl by R. Phillips
6. super heroic minimalism by Jonah Block
7. The Solar System by Steph Says Hello
8. The Infinite Doodle by Erica Strandberg
9. Honey Bear by Danielle Furman
10. Imaginary Exhibitions III - Masks of the World by James Barker