C.M.
Hi, J.!
I am a mom of three sons, and we used our largest cooking pot and turned it upside down when we were eating at the dining room table. To keep it from sliding, we used one of those rubber (waffle design) placemats that we purchased at the Dollar Tree (everything there costs $1.00). It was easy to clean up.
For safety reasons, I would recommend one of the booster seats made for use in the car. If your budget is tight, look around at the kids consignment shops, or thrift stores in your area. I've seen the booster seats for $3 - $5.
Ebay is another great place to find good deals. Happy hunting!
C.