Split the children into four groups and then give them an object, wooden spoon, tray, colander is a big success, any thing you want. They have to do a sales pitch for this object (like a tv ad), but they don’t sell it what it is, they have to pretend it is for something different. (this can be an interesting adult game too!)
They need a price, a place they can buy it from and any deals. Anything funny to get them sold.
Then you have a judge who decides which they would buy, based on the adverts, not the product.