The Trout Museum of Art looks like a typical small art museum. While it is not geared toward children, it could be interesting for them, especially if you have one who is interested in art. I think I would be most inclined to try one of their events that are more specifically for kids. There are regular family art project days as well as performers that they bring in, called the Young Audiences series.
Trout Museum of Art
Fox Cities Building for the Arts ~ 111 West College Avenue ~ Appleton, WI ~ (920) 733-4089
Admission: "Varies by Exhibition" (?) but at the moment it's:
$6/Ages 11&up, $2/Ages 5-11, Under 5 free