Last night I did manage to take down the flannel sheet from the wall and wash it. I am definitely going to use it for the batting in the tablecloth, as it is the only flannel I have in the house, and I am trying to use what I already have whenever possible.
I sewed two of those light blue blankets together and pinned them up in place of the flannel sheet. I think they will work out very well, as the surface is very felty and blocks should stick pretty well even without pins. They werent sticking to the previous sheet anymore, as it is getting old and the surface was wearing off.