I don't see anything wrong with eating picnic style, or maybe you can have booster seats on the floor so they can get into them on their own and it will keep them sitting nicely while they eat.
Are the kids too young to transition them out of the playpens? If not, maybe the mats from inside the play pens could be put on the floor too?
I feel your pain, I have a heel spur that developed during my last pregnancy and it hurts to walk, but I'm on my feet all day! I should be resting it, but it is very hard to do.