Veolia Environnement Wildlife Photographer of the Year has an under 18s section. This year's comp is closed, but the next one begins in January.
Tesco (big UK supermarket) have
kids_challenge_october for up to 15 years old.
and with this one you can win an SLR
First ever ePHOTOzine Junior Photography Competition
I'm not endorsing any of them, just competitions I came across.
DHG
edit-- I just had a look at your website, there are a few nice shots in there. The rhino is particularly good. The "ghost" photoshop is pretty good too. Keep up the good work!