DS2 - 9mo and crawling lots - gets put in leggings a lot of the time, as most trousers or tracky bottoms have a waist that's a bit too big for him so they end up sliding down his legs while crawling, an he just crawls right out of them! I get them from H&M, Polarn O Pyret (sale only, too expensive otherwise), and Gap do some nice ones, some have feet on too so no problem socks.
I hate seeing little girls trying to crawl about in ridiculous frilly dresses that they keep kneeling on. I'm sure that's why DNiece didn't bother with crawling - probably got fed up tripping over!