The parents find out what happened to their kid and are putting out mouse traps and getting out dangerous stuff to harm the rat.
The rat is panicked and runs out of the house.
“Those mean humans!” The rat thought. “I will show them what it’s like to be pests!”
The rat starts shrinking the house but then out of anger gets overwhelmed and shrinks every human in the neighborhood too.
Humans began to panic. The rat was proud of what he did and grabbed a few humans and put them in his mouth and chewed them to a pulp and swallowed them.
“Armageddon to humans is rather fun” the rat thought and went to look for the other rats to help him take over the world and maybe even feast on more shrunken humans