He's having marital 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 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.
After he blows up the kid in the next movie, his family leaves him. He changes his name and becomes famed superhero The Atom.
Okay hear me out! In the second movie Diane is well into her forties, and while it is not unusual for woman to have children at that age, she and Wayne had only just reconciled at the end of the first movie (they certainly weren't planning to have another child!). This might also account for the Thompsons disappearance in the sequels (the parents wouldn't want their son to face a paternity charge, or alternatively the Szalinskis moved because they were worried that the Thompsons would try to gain custody of Adam). This would make little Russ a Disappeared Dad and he and Amy Star-Crossed Lovers. Another thing to consider is the fact that in the third movie Adam and Wayne come to a slight disagreement over the fact that Wayne wants Adam wants to play baseball. Sound familiar? Not only does this parallel an argument between Russ jr and his father but it also raises the question who did he inherit this from? Its been shown again and again that none of the Szalinksis know crap about sports and while Russ jr doesn't either both his brother and father do, in fact it was Ron's love of baseball that kick started the events of the whole series! Whose to say he didn't pass it on to his nephew? There's a moment at the beginning of the second movie, during Amy's cameo where she says goodbye to her father and she's spoken to her mother and Nick yet she says nothing to or about Adam but the look she gives him is one of longing and sadness, its a blink and you'll miss it moment but its there!
Let's just say the military had to be called in for the Imagination Institute incident, and leave it at that.