Just got status matched and now AVIS President's Club status. Picking up a car mid Dec in SFO. What can I expect in terms of upgrades/cars available? or other benefits to take advantage of?
Not much, so don't get your hopes up. It depends on how generous the location wants to be.
I have rented "Intermediate SUV - Toyota Rav4 or similar"
With an intermediate SUV reservation, you'll be lucky to get a standard SUV. While it's possible to get something nicer, upgrades like this are hit or miss. You're much more likely to get a fullsize or premium car with a midsize reservation.
I will also need two toddler car seats - any chances to get these included?
Effectively, zero chance. Status isn't as prestigious or beneficial with Avis as many people assume it to be.