Let a 5 yr old experienced traveler fly as UM?
O.k. Don't see a problem there.
Teach a 5 yr old to use a credit card?
Nightmare in the making! For one thing, I don't that's legal anywhere in the U.S. was just recently able to get my teen a preloaded MasterCard connected to my PayPal account; I believe the minimum age for that was 12 and requires parental controls on the account.
If the child's trip is set up correctly, there should be no need for the child to have/carry either credit or large amounts of cash. The whole purpose of UM travel is to be the safety net.
p.s. she probably already knows more about the cell phone than you think