Amiah had a milestone week last week. She started crawling, cut her first tooth, started sayin "dada" and began pulling herself on everything! This includes her crib! So when she cries for me in the morning, she's standing up on hanging onto the edge (hope she doesn't bite it) and it makes me so nervous! Should her little head be able to peek over the edge? Just how low does this mattress need to be? Any advice? thanks!