FinGrabber is correct, it is 40 feet for the "Junior" qualification.
My understanding is that the limits are 40' for ages 10-11 and 60' for ages 12-14. Above the age of 15 is the same as for adults.
"A pre-certification, 20-hour course for kids age 10 to 15 teaches basic knowledge and skills. Completing the course earns the graduate a PADI Junior Scuba Diver certification card, so dives down to 40 feet may be made under the supervision of a certified divemaster, assistant instructor or instructor."
Personally, unless they were fully trained I would keep them in the 5 - 6 foot range. They won't care (or even know) how deep they are. If anything bad happens their first instinct will be to kick for the surface with breath held. Using some sort of surface supplied air would be best then they can't go deeper (or further away) than the hose length.