With my nine year old I said he needed to pass off one level before he could play his regular video games. He just took it from there. There is a concept of points for doing certain things. I didn't think much of it but he keeps track and somehow he cares (something like karma here I suppose).
My 14 year old had a very different reaction. She just uses it to fill over gaps in what she is doing at school. I have a degree in math but no matter how gentle I try to be I think she feels embarrased to come to me for help. Maybe I get too excited and try to explain too much. Either way she is keeping up much better now.
My six year old watches for a few minutes and then breaks into song and dance. I'll just let him be for a while.
In a related question, do you use OpenDNS or any other access controls?