This is basically the question My rights if seated beside passenger needing more space wanted to ask or perhaps should've asked but of course the entire situation sucks. Enough rambling :)
So you board the plane, sit down, buckle in and someone of size sits next to you. It's uncomfortable. Perhaps they are blocking your access to the aisle, too. How do you deal with the situation? Asking a flight attendant can be tricky because you perhaps do not want to yell over said passenger that "hey, could you please throw this person off the plane, they are too big" which is essentially what you are asking. And yes, the attendants totally should do it on their own volition if said passenger violates rules but no one wants a conflict... Standing up and finding an FA is also tricky because boarding is chaotic and especially with low costs very time limited.
So how do you deal with this situation? Unlike the parent I am painfully aware if you don't deal with this at boarding time then you are done for, the attendants can't do anything once the plane closed the doors and compensation won't happen.
And yes, I am just as painfully aware this is a bad bad situation for the person of size, too. see this blog post :(