It really does depend on the wheels/tires.
52/14 is what my son has run for years. It is what I have set every junior I can up with. Getting a junior cassette and getting the right chainring can really help a lot of things. My son has 10 full gears and 2 full chainrings.
Every year at the first few races I have officials sending dads over with that scared look on their face "they said you need to block few out." and everytime there's at least one junior who has a 50t crank and an 11-25 cassette. "Sorry - your limit screw won't let me block out the bottom 3". At least if I am in the shop I can find a longer screw.
Seriously though just put 26' on the ground and check before you get there. With the right pressure in the tires.