Rowan just started crawling this week. Up until now she's been focussing on communication and fine motor skills, and just kind of let the gross motor skills hang. Also, she HATES intermediary steps (was the same with sitting) and clearly just refused to do it until she could do it right. Went straight from not crawling at all to full on all fours crawling in the space of a single day. No army crawling, no backwards, etc.
Not all kids crawl, and even without the correction for premature birth, your kids are still well within the range for crawling. I wouldn't worry if I were you.