Is there hot tub covers made that can be sat on. My teenagers and their friends seem to be always sitting up on the hot tub.

They can actually order you something called a walk on cover. They are pretty strong.

Not sure but you should check online or cut a piece of plywood to fit

No, they are not supported in any way.

They are not made to sit on the children should be asked not to sit on them if they dont obey the rules they can go somewhere else

Call the shop they may.

They are all styrofoam. Keep your kids asses off of it