Is this ok for my bunnies?

i have 4 rabbits that we recently adopted i feel so bad for them i don't even think they know what timothy hay is or if they do they haven't had it in along time you should have seen the happy jumps they was doing with hay sticking out of their mouths anyway they have a outside hutch that they go in while the sun is going down and half of the night i feel bad because its wire bottomed (my husband is building a better one this weekend with wood floors and a small open with wire along the back) anyway i put a large piece of cardboard on the floor in there so they can rest their feet if they get tired...is this ok? its just a plain piece of cardboard no writing no stickers nothing