I was going to write almost everything Amy&Dan did. A 10:30 departure from DL is very risky.... Are you driving and checking in a car at LAX, or are you taking the Bus or Supershuttle which may have additional stops? You might check the website for your airline and see what they recommend for how long to get to the airport. LAX can be a madhouse for some airlines. Sundays is a bad day for some airlines, and less so for others. I remember being in line for Southwest checkin around an hour one time at LAX, and that didn't count the security checkin.
The last time we went to Goofy's Kitchen, we ended up using the full 90 minutes for our dinner. In the past, I've never spent more than around an hour, but, as pointed out, you need to add a little time for your check-in at GK. If you do an 8:30 reservations, you should have plenty of time to eat, but not necessarily time to visit all the characters. An 8:00 reservation would be better. If you don't use your full 90 minutes, or if you decide on an earlier reservation, you can stroll the grounds and shops a bit before leaving.