Hi Erin, I started my oliver + s sewing with a 3 month old and started with the school photo dress, which actually was fine when she started crawling as it is quite fitted and she is long so it never caught. It was the same with the hopscotch dress and the seashore sundress, that was fine. The bigger problem I encountered was that she put anything with collars in her mouth!
She had an ice cream dress while crawling which was great, though I recommend making an adaptation to increased the head hold size if your toddler has an ample head!
I found that the less fabric below the waist the easier, i.e. it could be a dress to the knee-sh, but not if it was wide and blousey, as that caught. A narrower dress like the school photo was fine.