I've used it with 2 kids, more around age 6 or 7. Personally, I think it's boring as all get out, but the kiddos were excited to be learning another language. And if you want no Christian content, well, then, it's pretty useless, since you're memorizing lots of prayers in Latin.
Latin For Children is planning on coming out with a book for younger kids, stuff like songs to build vocabulary. Overall, the LfC stuff is relatively fun (then again, watching paint dry is probably more fun than the Latina Christiana/Prima Latina books).