You do realize that you have to install it for each user, right? And that it won't work for "mail-only" (mxxxxxxxxx) users? The install will only work for the username you're installing SpamAssassin under. This is discussed in some other threads here.
There are a few other howtos (google for SpamAssassin and Dreamhost)... however all assume a certain amount of knowledge, though.
Depending on your childrens' ages, it may be best not to provide them with direct email access, or to only allow email from known addresses (aside from maintaining this server-side, some mail clients have an option to only display mail from known addresses)...
You may also want to look into various client-side filtering options.
I am pleased to announce that we are finally actively working on some (long overdue) server-side spam and virus filtering options, which will hopefully be effective for the majority of users; there should hopefully be some beta tests soon.