Piece of cake! She has one who can go to the restroom by himself and only one in diapers. I've flown with three in diapers (well, one was PT'd but he still slept in them at night so he had to wear a pull-up in case he fell asleep on the flight). They were 4 mos, 2 & 4 years, flying from Europe to California with a connection. Many more times I've done this trip with two in diapers.

I survived so she should too!

P.S. The letter is not always required. I've never needed one. Check the thread on this subject and be very careful that if she does need one, it conforms to the specifics of her destination and transit countries!