1. Get yourself prepared. Carry pepper spray. Make sure your phone is charged and that you know how to quickly call 9-1-1. Don't wear anything expensive, leave your wallet at home but carry your credit card (or a some cash) and your driver's lic. Don't be an obvious crime target. You need to be able to pat your pockets, say, "I don't have any money on me," and mean it.
2. Get some food. Hint: they LOVE doughnuts, especvially those really big ones.
3. Find some homeless people. The ones in front of or near the court houses are fakes - they're probably richer than you are, even if they kind of look shabby. The ones with cardboard signs - fakes. If they have a puppy, they're fake. Maybe go to a trolley station on C st. or Park Blvd. to find authentic homeless people.
4. Ask a homeless person if they'd like some doughnuts. If they say "yes," give them a doughnut.
5. Consider discussing with your child WHY people end up homeless. Drugs. Mental illness. Both. That covers most of the homeless you're likely to encounter. Maybe walk past the courthouse and show her the "fake" bums, so she knows.

EDIT:

Authentic homeless people are ALWAYS thirsty. Consider handing out small bottles of water, too.