Sorry, but I don't think is possible to block MSN to add emoticons...
What you can do is uncheck the opition to Show Emoticons and Show Personalized Emoticons....
(See image attached)
But remember that if she is smart, and know how to change opitions, she can return the opitions.... And her friends can help her discovering how to do it...
Unfortunately, as a person/child is on internet she is taking the "risk" to see some offensive things...