In reply to CharlesE:
I am a girl and started climbing at the age of 8, I trained 3 times a week with climbing coaches from the ages of 10 to 16 and got really strong just by climbing and doing exercises such pull ups, press ups and lots of core to work the abs but never did any weight training! It's really bad for kids that are still growing, it can do a lot of damage as Craig said and I know a few girls that started doing weight training too young and injured themselves really badly (messed up their bone development and can no longer climb)...
I got quite good doing this kind of training and even did some competing (at a national level) so I wouldn't worry about them getting strong; if they keep doing lots of climbing they will get strong really fast, no weight exercises required!
I hope that helps and I hope they keep enjoying it!