His pull out game was good

In the novel, Lolita tells Humbert that Charlie (the boy at summer camp) would have contraception with him at the camp (his parents must've been working in the medical field) and that Charlie would also turn to previously used contraception (i.e. used condoms), or something along these lines. So this explains why the girls at the summer camp would not get pregnant. However, I think most of what happened sexually at the camp between those kids wasn't necessarily intercourse - 12-year-olds probably fooled around with their genitals and made out and lighter stuff like that. It is implied that most of them knew sex ed beforehand, for coming from rich, educated families, and Lolita was taught sex ed by Humbert using manuals, short after they began their affair.

I'm assuming Humbert was sterile as he never got any of his women pregnant (going by the novel). The "game" at summer camp with Charlie wasn't intercourse, it was fellatio.

Also the boy at summer camp was banging all those girls. Wouldn't the parents get suspicious when their perfect little daughters came back from an all girls summer camp pregnant?

.because author Nabokov decided not to have his fictional Lolita get pregnant.