I'm a former Flight Attendant and I used to have lots of unaccompanied minors on my flights.
It's a perfectly safe way for them to fly. They will be required to do so since they're too young to fly completely alone.
Their tickets can't be booked on line. You must use either an agent or go through the airline.
Shop around if you're buying the ticket as UM fees can vary. Also, check with some airlines as there are booking restrictions (no night flights, for example). Check carefully as some might have higher fees but two siblings together only pay one fee. This might work out cheaper. Read the small print!
No, not all airports give out "gate passes" so that one adult can come with them to boarding. You can ask and/or find out.
They do not have an escort on the plane. We check on them but they sit by themselves. At the airport, a ground agent takes them between gates or to whomever's picking them up.
Make sure their electronical items are fully charged and send them with snacks and some pocket money.
Hope they have a good time!
Former Flight Attendant, 13 years, 2 companies
mom of 3, ages 9, 7 & 5 (not UM's yet but would if they needed to...)