Mattress pad, cushy as possible

[katenyc]

My dd sleeps in a Sorelle crib, on Lands End sheets. I'm not sure what kind of mattress is under the sheets, just that it is very firm and has a waterproof top. She seems a lot more comfortable on OUR bed, and I was wondering if anyone had advice for a mattress pad (since the mattress itself won't allow messes to soak through, the pad doesn't have to be waterproof). Fairly cushy, if possible, but not so thick that it creates problems with the sheets.

Hope this makes sense!

Thanks,

Kate