Are picnic tables generally made out of treated lumber or yellow pine?
Most Picnic Tables are made from Treated pine, I have heard that the treatment that prevents termites from damaging the wood is also harmful to humans and pets. If you plan to use treated pine I would suggest a sealer plus paint or polyurethane to seal all toxins in the wood. At least on all surfaces that come in contact with people. It is a great material to build decks and picnic tables and when screwed together will last a very long time. If cost is not a problem you might try the new composite material they now use for public parks etc. It is a non-toxic material made from recycled plastic and recycled wood. It cuts and drills and paints very similar to wood and they say will last longer. Best of luck.
