A little web research, Pinterest searching, and a little craftiness brought me these.
3 foot by 3 foot (for each twin) in the bedroom
I bought two 8 x 4 sheets of foamular insulation sheathing, R4, 3/4 inch thick. Made by Corning, I believe. Cut to size. For the living room I actually used a medium-weight drop cloth for the fabric. Just laid the board on the fabric, stretched and folded it over, and used my staple gun. What I DIDN'T know is that the staples may come right out of the foam/material, so after I did the entire board, I used duct tape to secure it even more. What? Nobody sees the back? Then once we figured out the exact location we wanted it, we nailed it right into the wall in the corners. By accident we had it on a stud - which would be great if we were posting heavy things on the board. But come on, how heavy is this really going to be? It's so lightweight it's not even funny. But hey, it's not going anywhere now.
For the kids, I cut the other board into 3 x 3 squares. I had purposedly bought extra chevron fabric when I "made" the bedskirts for them. I say "made" in quotes because I got the fabric, Kim helped hem it, we placed it on the bottom of the crib slats with velcro and viola! (See below) So I pulled out that extra fabric, followed the same steps, and were good to go. Actually, the fabric was much lighter so I didn't seem to need the duct tape.
Starting to look like a real room now, especially with the paper letters I spraypainted with glitter paint over Christmas! Good thing since they turn 9 months this week (what?!) and we've been in this house for 4 months. It's not like I haven't been busy though...