Snazzipants signature naps are a fitted so yes they have a cover over the top, but you don't need to change th cover at every change unless it gets "exploded" on. I found my girls only used one cover per day when we were using flat naps + covers.
Snazzipants have a great sizing chart on their website, and if you email
Pippa she'll be able to help you out with fit etc. too.