My son (now 15) flew as an UM for years, and never had a problem at all. It's harder for the parents than for the kids!
My only suggestion is perhaps to see if you can seat them together, with an empty seat between them? Although there is the risk of fighting (I have two boys myself) I think it is also comforting, especially for the 5-year-old.
I would also go over all the rules again with them pre-flight, and discuss "rewarding good behavior" in advance.
Let us know how it turns out!