Mr. Szalinski was originally planning on ridding himself of his family.He's having martial problems, and leaves the children alone in the house with a dangerous ray behind an unlocked door. The baseball did not actually turn on the machine - it was supposed to turn on when it detected movement in the room. The baseball actually "fixing" the machine so it actually shrank things was what brought him back to reality from the edge.
The Szalinskis settled with the Thompsons out of court for a portion of the profits from the shrink ray.The Thompsons would have had a very solid case for a lawsuit (no lock on the attic door, the ray faced said door, an unquestionably dangerous machine being built in a residential neighborhood, leaving the lawnmower out where a minor could use it unsupervised), but evidently they never sue. They must have shared the profits or even entered a full partnership. The big turkey dinner at the end is to celebrate the first profits coming into both of their pockets.
Mr. Szalinski later adapts the machine into a costume.After he blows up the kid in the next movie, his family leaves him. He changes his name and becomes famed superhero The Atom.