I was told that the open house was pretty informal. The Admissions office even encouraged me to bring my young children....they don't know my young children

Having said that, it is all relative. If you are used to slumping around campus in cut off sweats and tight layed t shirts- I would suggest something a little more formal. Maybe something you would wear to a family gathering. I do not think that you need to wear a dress/skirt, unless it is what you are comfortable in. If I go, I will likely go for something comfortable, washable, but not jeans. My generation always wears black...we feel we can't fail in black.
Most importantly, come prepared to socialize, ask questions, do your research on the school and their programs that are of interest to you so that you can ask good questions and present your best, most confident self. That isn't some fake representation, just be true. I think they are so used to seeing what people think they want, that when someone shows up being genuine- they remember them!
Good luck! Enjoy!