Unless things have changed, you can get at least one extra key, by asking for it...at least we did, at SSR, 3 years ago...we were a party of 5 (a mother-son combo, and a mother-son-daughter combo) in 2 studios...two of the kids were very small (5 and 2) and the rest of us were (are) adults. We each wanted access to the other's studio (we traded off childcare so each of the adults (a mom and two grown kids/siblings) could have some one-on-one time, w/out the tiny ones).
SSR was perfectly willing to give us the extra keys...we didn't put charging privileges on them and we had separate park tix (APs for 2 of us and hoppers for 3)...