"You have only paid to reserve your seat, if my child is seated next to you it is not your affair or concern, or responsibility. I'm not paying an unnecessary fee because someone might not want to sit next to my child. I will ensure she is well-behaved, if she starts to vomit (unlikely) I will come and take her to my seat. But as her parent it is up to me to judge if she can sit apart from me, and I judge that she can."
I'd be absolutely fine with this, ThisIsOurBlanket.
I'd also be fine with being asked to move, as long as it was done politely and as long as I didn't end up in a worse seat than the one I had chosen. But if someone was being aggressive or rude and entitled (as I have seen in anecdotes on similar threads), then I would be a lot less likely to be helpful and accomodating. Or if, as I have seen on other similar threads, someone said that, if I didn't move, their child would sit next to me screaming and vomiting the whole way - as a threat to get me to move - again, I'd be less happy to be helpful.
If you know that your child is likely to become distressed, if seated next to a stranger, or if you know they are likely to become airsick, and will need assistance, then it is your responsibility as their parent to make sure you are seated with them, so you can give them the care they need, and if that means an extra cost, then I am afraid you have to suck it up. Blackmailing someone into moving is not a valid solution to your problem.