In HHP's (and purebreds) you can show kittens as young as 4 months old (on day of show). After 8 months old, they have to be neutered/spayed to be shown in the HHP classes. And they can't be declawed for CFA shows. TICA and ACFA allows declawed cats, but neither association promotes declawing.
Out of your cats, choose the one that is most laid back and likes people. One that is not really scared of noise. Cat shows are quite noisy at times
I'd attend a few and watch before you put them in the show - you can get tips/hints as to how to set up the cages, etc.
Also you need to condition your cats to be bathed. They have to be totally clean and groomed well if you want to win ribbons