There most certainly is such a thing as an expired car seat. They have expiration dates stamped on them. Times change, safety standards increase, and that foam inside doesn't cushion your child as well once it's old and compressed. You can't tell by looking at it.
This is what I would do, personally ... I would go to
Walmart and buy seats. Two brand new Cosco Sceneras would be about $80 ($40 each) plus tax. They are easy to install after a quick read of the manual, and accommodate rear-facing infants as well as forward-facing kids up to 40 pounds. Booster seats are even cheaper if that's what you need, in the $20 range.
When I was done, I'd drop them off at a Goodwill or other thrift store ... there must be some of those near WDW. While you shouldn't buy used seats due to not knowing their history, some people do, and someone buying my very-lightly-used seat would actually be getting a good one. Plus it saved me a little money anyway. Yes it would take a bit longer going to Walmart to get them and all, but $40 cheaper plus helping out a stranger down the line.
If you do absolutely need to rent one, inspect them all carefully before choosing. Check all over the plastic body for cracks. Lift the cover a little and check the foam for cracks or missing chunks. Find the expiration date, which will be stamped on the plastic body, usually on the underside or the back. It will probably be molded into the plastic (so not a contrasting color or anything, look very closely!) and say something like "Do Not Use After DEC 2012". If there are any cracks or dents in the body or foam, or it's expired, don't use it. And I would point those issues out to the people working there.