According to "Trivia no Izumi", a popular trivia show on Japanese TV, the reason there are only kids in Peter Pan's group is because it's against to rules to grow up and Peter Pan kills any kid who does.
I thought this was pretty interesting, not that Peter Pan isn't already without its controversy, but, before I posted I looked it up and interestingly, the book they showed on the Japanese TV show does not match the one available on the net.
The Japanese program displayed it like this
The boys on the island vary, of course, in numbers, according as they get killed and so on; and when they seem to be growing up, which is against the rules, Peter thins them out; --- a grown-up dies; and Peter was killing them off vindictively as fast as possible.
That's actually out of context as they are several chapters apart. The second part actually says:
he was so full of wrath against grown-ups, who, as usual, were spoiling everything, that as soon as he got inside his tree he breathed intentionally quick short breaths at the rate of about five to a second. He did this because there is a saying in the Neverland that, every time you breathe, a grown-up dies; and Peter was killing them off vindictively as fast as possible.
Clearly it's a figurative statement. He isn't actaully killing them, he's imagining that he's killing them.
But, the first part does appear to say he kills them if that's your interpretation of "thins them out".
What a guy 😛