We did it a few years ago so it is possible that the food/show may have changed. We did not enjoy it that much. We thought the buffet food was lacking as was the show. It is slightly audience participation so that might be fun for the kids. At the end of the show they have you get up and stand at different ends of the room to show who you thought the "murderer" was. Then they picked some one to be the "winner" and gave them some plastic hand cuffs as a prize. (At least that was how it used to be done.)
I suggest that you go to the Sleuth's Murder Mystery Show. It is sooooooo much better. We have been several times. It is off property and a slight drive from the WDW area (it is located on Universal Blvd in the I-Drive area). The food is plated as opposed to buffet and you have a few options to choose from. We found this food to be much nicer than the buffet at the Grosvenor. Also there are different shows you can see, I think they have about 10-15 different shows that alternate show times throughout the week. These are also murder mysteries, but they have the best actors who really know how to get the crowd going and involved with a multitude of pop culture jokes and references that make the shows hilarious (they are slightly improve I think because I have seen the same show "Island For Sale" twice and while the base plot was the same, the audience interaction really changed the feel of the show). At this show you are seated at a table of about 8 I think so if you do not have a large party you will be seated with strangers. This was fun because the nature of the show causes you to interact with the people at your table to come up with a question during the interrogation phase. You can also print a coupon for the show off of their website.
I would highly recommend Sleuth's, but wouldn't again waste my money at Grosvenor's.