But how do you know for sure that these particular younglings were even at the Jedi temple when Order 66
was given? How do you even know that Anakin slaughtered every single one of the Jedi that was in the temple at the time? We saw maybe a dozen younglings hiding in the Jedi council chamber... Where were the rest of them? There had to be much more, and it's possible, if not probable, that some of them actually managed to escape in time. Who knows, maybe most of them were taken to safety long before Anakin reached them, and those poor kids at the council chamber were just the unlucky ones?