I've encountered a couple of people using a program or service (I don't know which it is) called "Spam Arrest."
The way it appears to work:
No mail is delivered unless the sender is in your address book.
Any "unknown" sender receives and email requesting you to click a link in the email to verify that you sent the original message. (The address for reply was at spamarrest.com.) When you do the verify, the original message is delivered, and presumedly your address goes in their addy book(?). Since most Spammers won't bother answering mail, it probably is effective in cutting out most of the slease; but people you want to get stuff from may (if they're sensible) object a bit to having to click links in email from ????? senders.
It may be possible to turn off the "verify" circuit, so that your child would receive email only from people in her address book. She'd have to know who might be sending stuff so she could put them in the addy book before she could receive mail from them; but this might not be a real problem if she's only trading email with known friends. And nobody would be able to give her addy to anyone else, since she wouldn't be able to have the "unknown" sender addy in her book if she wasn't told in advance.
I don't really know much about this system except that a couple of shady characters have used it on me. It seemed to work, but I'd suggest careful investigation before signing up. There probably are other similar filters, or possibly a setting on some email programs to admit mail only from those in your address book ??????