Regarding paint, at Home Depot, (and many Lowes), they have mismatched paint and it sells for $7 a gallon. Of course, it comes in all colors but for the primer paint, it would not matter. With Kilz, you have to be sure to cover your arms too, it can get into your pores. I would say, try the children's tiles, or colored tiles, not the carpet tiles.
See what is in them first. They would have to be suitable for children. Then price them thru a carpet place first, then try BJ's or a place like that. I just got some at BJ's for my laundry room. They come in about 2' square. I think you could get them in any color. A light color would be great. You could always hang mirrors for more light. Have fun!