We do a similar relay at our Fall track and field. The PE teacher runs it, but the clothing is usually "scarecrow" or "farmer" to go along with the fall/harvest theme. So overalls or BIG jeans, plaid shirt, big read hankerchief in the pocket, etc.
My only concern about a clown costume might be the durability. Most costumes are so flimsy, would they survive?
Also, at our community 4th of July party, they have swimming events/contests for the kids. It doesn't run continously the way a game does, but with a schedule. So 1:00 is one event with two or three age categories. Then 1:30 would be another. Depending on how long it takes, there might be gaps of time. The kids consult the schedule and show up for the ones they want. Something like that might work for your event.
i had thought about doing relay races but was not sure how to actually put them into practice. can you keep enough kids interested? how long will the game take? will it cause long lines? how would you run a race if you have 3 kindergarteners and 2 5th graders? we couldn't figure out how to make it work so we decided not to try it last year. if you come up with any ideas i would love to hear them. good luck.
One that I thought would be good for a Halloween event would be a clown race. You have two teams of 3-5 people. Each team has a suitcase with a clown costume in it. At go, the first person on each team runs to the other side of the room, opens the suitcase, puts the costume on and runs back. Then he takes it off and puts it into the suitcase for the next team member. Everyone would get at least a consolation prize.
There are others that would work too. The reason I thought of it is because of the ladder race. That seems to do pretty well at carnivals.