The easiest thing to do is find a decoration pack that contains multiple decorations that you can use. You can get decorations that hang from the ceiling, or table decorations. Outdoor parties allow for fewer options, since there is not likely to be a ceiling from which you can hang things.
If the party is indoors, peel and stick wall decorations are also a nice touch. You can get a set of Disney Cars decals rather inexpensively, and simply discard them after the party, without making holes in the walls.
A nice main decoration is a large, theme consistent balloon. These can be added to the table, and allowed to float above the children, or can be taken away by the children as an object with which to play. Finding helium, which is safe from fire, can be bought at a party supply sore in a small container. You probably will not need a large tank, so look in your location for the smaller ones. Or, you may be able to pay to have your balloon inflated by the party supply store.