Because of an unfortunate divorce, my daughter was flying UM on international routes to Asia at 5. I actually think that on long hauls, it is easier, both for the kids and for the crew, because there is more and better food, there are no charges, and thus less problems with money, more entertainment options, and in general more staffing. Her experience (now she is 11) is that crew are extremely solicitious with UM kids. On one Air France flight, she went to sleep in economy and woke up in a lay-flat seat in First Class.
Domestic US travel on the other hand...make sure she has enough money, and try not to book on busy dates, planes are understaffed as it is.