My 4yr old is a pretty good kid. She probably wouldn't do half of what I listed & she isn't gobby, but neither is she impeccably behaved, because, well...she is 4!
She probably wouldn't spill stuff all over you, but she would want to wonder around (thus getting you to move for her, if she wasn't on the aisle) she would need help with her in flight entertainment system. She would want drinks, snacks. Which would probably be packed for her, but she may need help getting them out the over head, she would also need help getting her books/stickers/toys etc. She did tend to drop a lot on the last flights we were on, so that is to be considered too.
She would actually see it as an opportunity to make a new friend & you would be told all sorts of interesting facts about Concorde (including why it crashed & doesn't fly anymore), about world war 2 & dinosaurs. Oh & why she likes Thomas. She would be upset if you asked her to be quiet, because she would think that you didn't like her.
My 9yr doesn't need anything special, she just would want to be with me. She would throw up, but she would be OK after that with her earphones.
I wouldn't expect a mother to leave her dcs, I wouldn't expect any one to move at all to be fair, but they would have to put up with my dc. They aren't badly behaved actually, and I would do what I needed to do to ensure they weren't a nuisance, just as I would if they sitting with me or in any other place in the world. But I would be limited as to what can be done depending on where my seat in in relation to theirs.