Like weddings, they range from very small and simple to very elaborate. Many will have a DJ and dancing/games. There is generally lots of catered food and drink. Your child will have lots of fun!

The Bat mitzvah will have a service where the Bar Mitzvah child leads the service in some combination of Hebrew and English. This is a major accomplishment and one they have prepared for over a year. To attend the service, dress conservatively like one would for any religion. Girls keep shoulders covered. Boys wear a suit and tie or khakis/blazer. Those services are open to the public, so you may attend if you are interested too. Following services, there is usually a little snack - think juice and cookies, but sometimes lunch.
After the service that day or evening, there is a party to celebrate. Some are simple some are extravagant. Kids usually wear the same dress from the service, often with sneakers instead of dress shoes. This party is by invitation only, so plan to drop off and pick up.
The hosts usually have someone chaperone the pick up, especially if late at night.

Please do a search. There is so much information already out there. And make sure they weren't invited to the service - that seems weird.

My kid has been invited to the celebration portion of a bat mitzvah. None of use have ever attended bar or bat mitzvah, so she's curious what to expect. I think the kid's whole class will be attending, but parents are not invited (which I wouldn't expect, because we don't know the family!).

Can anyone provide a bit of info for us?

Thanks in advance!