Much depends on the household of the friends. There are a variety of possible practices and traditions which you might encounter but not everyone has the exact same take on everything.
For example, if they have yet to light their Channukiah (the 9 branched candelabra) you might watch them do that and sing 3 blessings. They might say the blessings in English, or in Hebrew. With a tune or without.
They might sing Channukah songs after lighting (in English or Hebrew) and who knows, maybe even dance a little. There might be some coins given to the children (real or chocolate) and there might be gifts given. If the invite includes dinner, they might serve potato pancakes (with apple sauce or sour cream) or jelly donuts, or other foods that befit their particular geographic and ethnic heritage.
The things to speak to your child about range from the obvious (we had to stop our little one from blowing out the candles...she thought it was like a birthday cake) to the less obvious -- don't ask "where's the tree?" or "why are they lighting that fire and saying weird words?"
And if she doesn't like latkes (potato pancakes), she shouldn't feel beholden to eat them...more for me.
Chag Channukah Same'ach (happy Channukah)