The thing is, they cannot sit children that young unsupervised. They will have to move you around somehow.
The two and one year olds, MAYBE. The 5 year old? Sure they can.
The transportation forum is a really good place to look for info on this. Listen, that forum does NOT dust itself with pixie dust; they will tell it like it is. But they will also give you good info. Go there, do a search.
It's REALLY too bad you didn't know to book seats earlier than this. Right now you're in the hands of the computer, and you're up against people who DID know to book their seats. Or at least to call the airline and link reservations, in case they were made separately. When you get there, it's the same situation. And when you get to the gate, same situation. When you get on board, you are in the hands of other people who have their own reasons to sit where they chose earlier.
I don't know what airline you're on, but if you go to the Transportation forum and *ask* what you can do, giving the info on airline, etc, you can learn the truth of what you can do.
I've never looked into it, but on a similar thread on that forum the other day, people who are more expert than I am with flying said that children at 5 can fly alone...so you really canNOT *expect* that they will 100% be sure to put the 5 year old with you. You should, however, be able to get solid info on what you *can* do.