Hi guys. I recently inherited a dining set from my grandpa and it could use a lot of TLC. The chairs appear to be pine or other light wood, with plywood backs. The backs are simply black where dirty hands would touch the tops to pull them out. Some of the finish has worn off to bare wood. The table is in the same sad state.

I need advice on how to finish the plywood backs and the table as it is made of a plywood type wood as well. I probably have to sand them a bit to get the grime off as I can't seem to clean it with anything I own. So questions are.
1. what would be the best thing treat the table and chairs with? I.E. after the table and chairs have been completely sanded and cleaned, do I need to start with a sanding sealer? Recommendations?

2. WHat would be a good finish for the table? It will go through a lot of wear and tear with my kids around, so something to protect the wood from scratches, water, ect.