I doubt it was a small child. And I see demanding moms all the time. What I would have done completely depends on a number of variables. Was mom a demanding harpy or was she polite? Was it a timid looking 9 year old or a 12 year old who is taller than me whom mom probably still takes into the ladies room with her? Will I have to exchange my aisle seat for a middle or window seat, because that's not happening. There are several reasons I pay for the aisle seat. Will I have to sit next to someone spilling over into my seat? No thanks. Will I be seated next to the bathrooms? That makes me gag, so no. When we flew with DD, I always paid extra, even for early boarding. I didn't want to take a chance on being separated or having to rely on someone switching seats.
When we're at WDW, I can't ride a bus unless I am seated. If the bus is full and I'll have to stand, I wait for the next (empty) bus so I can be guaranteed a seat. Once I am in that seat, I don't give it up. I don't get on a full bus and hope someone will relinquish their seat for me. That is too much to ask or hope for. We make sure we get seats by waiting.