Very few children like clowns. They are unfamiliar and come from a different era. They don’t look funny, they just look odd. Children are much more happy with things stuck on the wall that have some sort of personal relevance for them, not some images that are foisted upon them by adults.
But it's not just clowns in children's ministry. We hang onto all sorts of bad ministry ideas just because they are easy for us to do, or we're comfortable with them, or they worked for us in the past. It just doesn't make sense. JM