What does Rex do with the kittens? How does this relate to how he and Rose Mary feel about their own children?

Rex kills the kittens.

Too many, in fact. We had rescued lots of cats since we tossed Quixote out the window, and most of them had gone and had kittens, and it got to the point where we had to get rid of some of them. We didn't have many neighbors to give them to, so Dad put them in a burlap sack and drove to a pond
made by the mining company to cool equipment. I watched him load the back of the car with bobbing,
mewing bags.

"It doesn't seem right," I told Mom. "We rescued them. Now we're going to kill them."

"We gave them a little extra time on the planet," Mom said. "They should be grateful for that."

Rex's actions and Rose Mary's explanation reflects their feelings about their children. Their parenting skills are almost non-existent.... they leave the children to fend for themselves and to be grateful for whatever attention they give them