What does the 12 year old boy do to get Will and Jim in trouble?

He steals from Miss Foley, and the boys get blamed.

'Police!' Robert threw one last spray of glitter at their feet, shut up his fresh-peach smile like locking an explosion away in a box, and shot away down the street.
'Wait!' Jim jumped. 'We won't hurt you!'
Will tripped him, Jim fell.
The window upstairs opened. Miss Foley leaned out. Jim, on his knees, held a woman's wrist watch. Will blinked at a necklace in his hands.
'Who's there!' she cried. 'Jim? Will? What's that you got?!
But Jim was running. Will stopped only long enough to see the window empty itself with a wail as Miss Foley pulled in to see her room. When he heard her full scream, he knew she had discovered the burglary.
Running, Will knew he was doing just what the nephew wanted. He should turn back, pick up the jewels, tell Miss Foley what happened. But he must save Jim!