Why not just do a few clear rounds, or don't they have them where you are? Or if you're embarrassed about competing against littlies (I wouldn't be, they usually win!) ask if you can go hc (that means you're going round the course, but you're not competing).
But really, as long as it's within the rules, I'd say just go in for whatever you like. Usually there's a restriction like 'horse and rider not to have won 1st, 2nd or 3rd in a jumping competition', but if that doesn't apply to you there's not problem. And if you do meet snarly comments, just laugh them off, and say "she's a brilliant little rider - she was unlucky not to win" or "I'll compete against someone my own size next time" or anything at all really...
Sorry, I can see this probably too late to be helpful. How did it go?