Embroidery Library www.emblibrary.com has some nice tiny designs.
check out your own embroidery machine first to see what you have on there. I have dog, cat, penguin on my machine and they are ever so tiny. Done in different colour ways, they look like different designs.
Pillow cases with their name embroidered on the hem, made out of special fabric. My great grand wanted Seahawks cases which she sewed herself. She loves pineapples so we found salmon background with gold pineapples for case and cream with gold pineapples for hem. My family lov es them I made andonce had a lady order 13 pairs for her family for Christmas. She provided color likes, theme likes. I made 2 extra pairs and she had trouble choosing her final picks.
PM me if you want dimensions for the 3 size cases.
You could do a pillow for each room. Just embroidery a design on fabric and use a pillow insert. That way you could match a design that would be significant for each child. Good luck!
It seems to me that I have received a lot of mini patterns from Designs by Juju.