Hey michaelr,
Welcome to the forum!
It may be hard to find a circus layout for sale, but a train set would be a good start. You would be surprised how quickly you can set up a layout/display and involving the kids with the construction is a learning experience too. A ping pong table is a good short cut to starting a layout or any smaller table to fit your available space.
I quickly searched eBay for a layout/display and a found 2 but they are not train related and considering you are asking in a model train forum I assume you would want trains in your display/layout, but her they are any way
http://www.ebay.com/itm/VINTAGE-Coc...241?pt=LH_DefaultDomain_0&hash=item4acf65e479
http://www.ebay.com/itm/Large-Circu...190?pt=LH_DefaultDomain_0&hash=item51b85c6a16
You can keep checking back with eBay, you never know what you can find.
I found this train set on eBay starting bid $180
http://www.ebay.com/itm/Lionel-Lion...6122?pt=Model_RR_Trains&hash=item2a36a6041ait + shipping It only has a few hours left before it ends but there are no bidders and it will most likely be relisted. The book value for that set is $155 in excellent condition and $190 New. That set is claiming to be new. It is from 1991 but those locomotives from that time period are of very good quality and American made built to last. There maybe others as well.
Here is a new HO set
http://www.modeltrainstuff.com/Bachmann-HO-Ringling-Bros-Ringmaster-p/bac-00714.htm that is from my local hobby shop and they have some of the best prices you will find anywhere on the internet.
How old are your kids? The reason I am asking is the younger ones have trouble with HO or smaller scale trains.
My grandsons are 4 and 5 and they do great with the Lionel O gauge, but they struggle working with my HO trains. They can operate the throttles but have trouble railing and coupling the HO cars. They have no trouble with the Lionel O gauge.
I hope that helps, if you have any further questions don't hesitate to ask and I hope you find what you are looking for.