IDK WHERE TO POST THIS

Aren’t mattresses supposed to be SNUG in cribs?! I feel like there’s too big of a gap between my mattress and crib. Like I can push it all the way down and over and there’s like 2-3 INCH GAP! How do I fix this?

(No my son won’t sleep with or on the blanket so get off that pony before you give me shit about blankets)