It could be flea bites. Or possibly spider bites?? Or, maybe it's a microscopic bug that hides in the mattress. If they itch really bad, I'd have them checked out.

Or, you can go and get anti-itch cream and put it on till they clear up.

Good luck, kiddo.

BTW, I'm in love with your Abby girl. I had Airedales growing up as a kid.