It doesn't hurt to add the request to your reservation and then mention it again at check in. In my experience it's about 50/50 as to whether it will already be in your room when you arrive (if you've requested it in advance on your reservation). Not sure where you are staying, but keep in mind that "crib" generally means "pack 'n play/play yard". Not many resorts have real cribs anymore. If you need a real one, you can guarantee yourself one by renting from A Baby's Best Friend or Orlando Crib Rentals.
Have a great trip!