i don'think that "crawler" is a good translation. long shot at best. "reptile" is your guy.
so... I get that "the little snake" might be too specific, but why was I told the right answer was "the little crawler" instead of "the little reptile?" Would I be able to talk about a crawling baby as το μικρό ερπετό?
No, crawl can have 2 translations in greek: έρπω=crawl (used for animals mostly) or μπουσουλάω which is used for babies. Calling your baby ερπετό is like calling your baby something along the lines of a chameleon or a snake... Crawler with the meaning of "he who crawls" is έρπων(masc)/έρπουσα(fem.)/έρπον(neut.) but it cannot be used for babies, the same way έρπω can't. Crawler with the meaning of "sycophant" is συκοφάντης. Crawler with the meaning of "obsequious person" is χαμερπής or δουλοπρεπής. ;)