one day is plenty
if your toddler likes water rides you'll be great. i had season passes one year and my daughter hated being splashed in the face so we did the few dry rides they have, and the activites, shows, and so on...
look on their calendar to see if a kids music band is comnig that your toddler likes...my daughter loved the kid concerts and shows there.
bring water shoes and pack lunch. you can eat the lunch in the parking lot there are a few picnic tables and also a few grassy spots. you can also bring lunch and eat it in there as well. bring candy so you're kid wont be sad seeing the other kids eating 5 dollar lolipops and bring popcorn for the shows
Oh if you get a locker, see if you can find someone leaving at the time you're going and offer to pay them the amt they'd get back for returning the key or a dollar more...then you're only paying that one dollar since you'll get back the return money when you return it
I'd stay two days and go to the please touch museum one day in philly and possibly the philly zoo. the kids museum is so awesome!--try and go on a weekday to sesame and also to the please touch or its packed!
OH ALSO if you want to get your kid their favorite character stuffed animal buy it in advance and hide it and give it to them there...their prices are sky high!