Yeah, but that's kind of retcon on Donna. Clearly, she was intended to be the same character as the previous Wonder Girl when she first joined. Though she's a weird sort of limbo, I'll grant, since there was a Wonder Girl, like you said. And even so, it was a while before she got a name (Not until "The Origin of Wonder Girl"- issue 22, right?) I seem to remember letters asking about her origin for a bit before that.
I don't remember Garth's name being mentioned in first appearance, but it might have been.
I have read some of the early stories, and they really were heavy on the parent/child aspect of the relationship then.