1. Everyone will gush, "What an amazing opportunity!" but few people will say, "Oh, yeah, that's going to rip a chunk of your heart right out as you wave goodbye to your baby whom you've spent years protecting from the world only to now fling this being into risk's open maw." It's OK to cry.
2. Even if your child is a hardworking young thing who's saved up to go on this trip, even if he or she is going through a school program, you're going to rack up some costs. Proper luggage. A passport. Extra socks. Minimize your expenditures by doing research via Lonely Planet or your friends who've traveled to find out what your kid will really need and what's superfluous. Borrow things, but nothing irreplaceable.
3. Yes, he or she will learn much about a non-American part of the world. Odds are he or she will also gain experience in the international skills of navigation, patience, making friends, solving problems in a foreign tongue: e.g., Which train do I get on? and What is this that I am eating? and Where can I wash my hands? (That last one may be a mother's wishful thinking. A more likely one might be, Where can I rent a moped?)
4. People in other countries are sexy, what with the accents and all. Also, the thrill of adventure is one of life's best aphrodisiacs. This is a wonderful time to talk about sexual boundaries, protection and awareness. If you haven't done that yet, it's time. If you're not sure how to start, hit up Planned Parenthood. The people there will help you out.
5. You will never be more thankful for the Internet. You'll want to download "What's App," a texting app that allows you to chat with your child cost-free via Wi-Fi. Even though the kid won't (and shouldn't) text you as much as you'd like, you will be fortified to know instant communication is at least an occasional option. Do not be surprised, however, when the texts you receive fail to give an update on the magical experience you've helped provide and instead look something like, Can you transfer more money into my account? and It's good. I'm trying to find a place to poop.
BONUS: While the apple of your eye is off gallivanting and gaining worldly knowledge, you will have a quieter house and in that quiet you will realize that someday it will be like this. While you will never cease worrying, you will, at some point, turn responsibility for your child's life over to that child, who will be an adult. You will have done the best you can to prepare your offspring for the world and he or she will go out and discover, on his or her own, things about travel and money and safety and love. It will be hard for you, but a small part of your mind, the part not consumed with fretting, will hear the quiet and be grateful for it.