What size mattress do your mini cribs take?
Our mini cribs fit a standard mini crib mattress size (approximately 38"x24") up to 3" thick.
With that being said, some mattresses will fit better than others. Much of the reason for this is the varying density between mattresses. Firm mattresses will generally provide the snuggest fits as they do not compress as easily. Although a mattress may be slightly smaller than another, if it is denser it will likely fill the space better.
To determine if your mattress is a good fit, the JPMA (Juvenile Products Manufacturers Association) recommends that there should be no more than two fingers width, one inch, between the edge of the mattress and the crib side.