Hi Katie!
Thanks for visiting the Disney Parks Moms Panel.
I'm super excited to know you're considering flying in for a single day visit to the
Happiest Place on Earth!
The answer to your question really depends on the size of your car seat although I personally have never seen a guest try to fit a car seat inside a Disneyland Resort locker.
I will tell you that
locker rentals are available in three sizes. The rentable large lockers (available outside of the Disneyland Park Main Entrance) are 18" wide x 25" high x 37" deep...Perhaps not large enough for an average car seat.
One idea is to bring a stroller with an attachable car seat. That way, you can transport your little one in the car seat during the flight as well as during your visit to the Parks.
Another idea is to use a third party company for a stroller rental. There are a number of companies that rent and deliver strollers to guests visiting the Disneyland Resort. If you prefer, you can pick up the stroller yourself at their office location. A couple of companies that come to mind are: Traveling Baby and City Stroller Rentals.
Finally, the Disneyland Resort has
stroller rentals that are convenient and very easy to maneuver around the Parks for a very reasonable fee.
I hope this helps.
Feel free to send any additional questions to the Disney Parks Moms Panel, where we're happy to help!
Have a Magical Time!
-Anabel M