If you are referring to the Garden Grill at The Land (or whatever that place is called currently - I still call it the Land Grill Room), we did that a year or two ago. There was a coupon for either AP or DC holders, don't remember which, to get it free.
Anyway, each guest got a sundae that was much larger than we anticipated and the kid got a special one made to look like Mickey or something (its been a while). Characters came around - Mickey, Chip and Dale are the ones I recall. Maybe Pluto too.
It was fun and we all enjoyed ourselves. I don't know how much they charge for this because we got it free, but if it isn't terribly expensive - think what you would pay for a large sundae and some quality character time - then I would go for it.
Steve
P.S. Why are you doing an ice cream social BEFORE dinner?