Hi Melinda!
Thank you for bringing your question to the Disney Parks Moms Panel. I just had a very similar trip traveling for Tinker Bell Half Marathon Weekend with just one day in the park with my little one!
If it is within your budget, I would recommend the park hopper ticket. You'll find the parks at
Disneyland Resort to be within 100 yards of one another so park hopping is a complete and total breeze! This way you can easily bounce back and forth and give yourself some time to take in the classics at Disneyland as well as all the fun at Disney California Adventure Park. If you are looking to just visit one park, I'd really go with your sons personal interest. At Disney California Adventure, you'll find all sorts of Pixar fun between the the Cars Land area, a Bugs Land, and two other attractions with Monsters, Inc. Mike & Sulley to the Rescue! and Toy Story Midway Mania! Some of the rides at this park have a height requirement that you won't find at Disneyland Resort though so I'd definitely look into that before visiting!
If you have more of a classic Disney toddler, Disneyland Park is where you'll find Peter Pan's Flight, Storybook Land Canal Boats and It's A Small World! I do think Disneyland probably has more for you to do with a toddler, but if your son is big on any of the Pixar films, it may just make his day to see some of those characters!
Historically, I believe the weekend is very manageable with crowds, but I did find this past year to be a little busier at Disneyland Park than expected, but still relatively low at Disney California Adventure. For parking, I would just give yourself ample time if you plan on coming before the race. I know there was some traffic for those Guests that left immediately after the race, but since you plan on spending the day there, this shouldn't affect you.
-Lindsey ºOº