I searched for Companion Fare scenarios and couldn't find this one... hoping someone can help.

My wife and child (4 years old) are booked on a companion fare over Thanksgiving. I am flying on a separate reservation, same flights. My wife needs to stay with family a few extra days. If she doesn't fly back on the companion fare reservation (her's is the original ticket vs. the companion one), will they still let a 4 year old check in? I'll be checking in with her and be her travel companion.

I messaged Alaska and the initial response I got was essentially, "If one person on companion fare doesn't fly, then neither can fly". If that's the case, so be it, but that just seems odd. They've already got the money and wouldn't they be fine with one person not flying? I'm not looking for any refund and would be happy to let them know in advance so they can try to re-sell a seat... I just want to make sure the 4 year old can fly. The fact she is 4 may not matter, but in my mind it's a compounding factor. Thanks for any help!