When Jeremy went to get his pants from the Radley's place, what does Atticus tell his son? Also, why did Scout and Jem sneak out when Atticus specifically told them not to?

That would be Jem, and he doesn't go to the Radley place to retrieve his pants. They are returned to him.

None-the-less, when Atticus sees his son without them that evening, he asks where they are, and Dill makes up a lie. Atticus' response, “Jem, Scout,” said Atticus, “I don’t want to hear of poker in any form again. Go by Dill’s and get your pants, Jem. Settle it yourselves.”

The pants are returned to the Finches' house folded neatly and Jem shudders when he sees his pants folded neatly. A quote from the text:

"Jem, Scout," Says Atticus, "I don't want to here poker in any form again. Go by Dill's and get your pants, Jem. Settle it yourselves."