I think it depends on the situation for me - in the one described here, I'd politely decline to give up my seat - because 1) the mother just took it without even asking and 2) the other seat was extremely uncomfortable. Otherwise, I'd give it up because I still remember with a good bit of gratitude an Australian gentleman who gave up his seat for me when I was little-
A flight was overcrowded and they put me, about 6-7 years old, about 10 rows away from my mom on an international flight. I was seated in a window seat by a young woman originally and was okay, but she then switched with an older man. To my 7 year old eyes, he was really creepy, I felt boxed into my seat as he knew the passenger in front of us and was loudly talking and gesturing wildly. I just sat there silently crying until I heard my name - my mom was in the aisle seat across from a the nice gentleman and when she explained what had happened and asked nicely, he gave up his aisle seat to go sit in the window seat where I was. The older lady next to me whispered that she was glad I was sitting next to her now so that she could put her feet up without worrying about her skirt slipping - it was a good flight after that!