I haven't been to one of the family picnics, I've only heard about them. I know the dress code is the same as for visiting. I've been told that the food is usually served in quantities that could feed a family of four.
They should have a menu available to your loved one, so he should be able to tell you what the menu is for it.
I'm not sure how it is at Two Rivers, but I recently went to a family barbeque at Warner Creek and I would guess it's similar. Yes the dress code is the same. Since it is invitation only the check-in is a lot faster. Ours was a barbeque and the had hot dogs, hamburgers, potato salad, beans, chips and cake. The nicest part was you didn't have to stay at the picnic tables you could walk around and visit with others my husband could introduce me to some of his friends and their families. A lot more casual than regular visiting. They brought out a cart with toys and games for the kids and even had a little inmate band. It was really a lot of fun. I hope you have a great time too! :-)