J.O.
Consider a pack 'n play for the younger one...they are deep enough that they cannot get out of them until they are older. And their legs can't get stuck. My son used one for overnights at grandma's until he was three. You can get them cheap on Craigs list. A toddler bed could work for the older one and they are pretty cheap at Walmart (my son's was $50) and use a traditional crib mattress. I wouldn't put a 1 1/2 yr old in a toddler yet bed though. Too much freedom!