My 17 yr old daughter will be traveling alone to volunteer at an orphanage in Kenya. Her itinerary takes her from Atlanta to Washington Dulles to Dubai to Nairobi (with a reverse of that on the return three weeks later). At what point or points (both to and from) should she expect to have to retrieve her checked bag and lug it through customs?
Flight to Nairobi, Kenya: Most likely, your daughter will go through customs in Nairobi.
Flight to Atlanta, USA: Your daughter will go through customs in Washington Dulles. She will have to get her luggage in Washington Dulles (the port of entry to the US), go through customs, then re-check her luggage.
When you check in at the airport, your luggage will be tagged with its final destination, insofar as the original airline is concerned. If they can check it through all the way to Nairobi (as you will want to do), then your daughter won't have to touch her luggage again until she gets there. In Dubai, there are usually agreements by which luggage can be transferred from one carrier to another, but if you contact the originating airline, you can find out for sure.
Customs must be cleared anytime you leave the sanitized portion of the airport. If you have no luggage, this is usually just a formality - the checkers can see you have nothing but a carry-on, and can inspect that if they wish, but usually don't. I stress "sanitized portion" of the airport, as once, going through Amman, I took a wrong turn down an escalator, and ended up having to purchase a $25 visa for Jordan, just in order to get back to my gate.
Note also, that even if your daughter has to claim her luggage, as long as she isn't exiting the sanitized part of the airport (and in Dubai, unless if she is leaving the terminal, she won't be) she doesn't have to clear customs until she gets to Kenya. As a general rule of thumb, you only clear customs as you leave the terminal.
Finally, Dubai is a major hub for many, many airlines. It isn't going to be confusing.
tl;dr> Most likely, she won't have to worry about Dubai - she'll just have to clear customs in Kenya.
Based on the information you've given, it's impossible to tell for sure. But, if your daughter's journey is booked on one ticket, and given the nature of Dulles and Dubai, most likely the luggage will be checked through all the way.
You could contact the ticket issuer or airline to be sure, or you'll have to wait until checkin and confirm then.