What it the new game the children make up?

You can find the answer to this in Chapter 4.  The name of their new game is "Boo Radley."

The way that this game works is that the children pretend to act out things that Boo does.  They get their ideas from all the crazy stories that are told about Boo.

By the end of the chapter, there is some conflict between Jem and Scout as to whether they should keep playing.  She wants to stop because Atticus caught them playing and clearly did not like it.

She is also nervous about it because she is sure she heard someone laughing in the Radleys' home when she rolled into their yard in the tire.