There are basic rules for this game, but you could adapt them in several ways depending on how long you want the game to last.
To start with, give every child a balloon to blowup (if you know the names of the kids at your party then put names on the balloons it will run much smoother or meet and greet all kids on arrival and give the kids a balloon and the parents a sharpie), and ask them to hold the neck rather than tie it in a knot. Place a target in the middle of the floor like a hula-hoop (or a large X for a Pirate Party), and on the count of three, everyone releases their balloon.
Whichever balloon is closest to the target, wins.
You could play this game over and over, and each time the target becomes a different prize, or you could allocate points for bullseyes, 10 points for 1st closest, 8 points for 2nd closest, and so on.
After a few games you could add up the points to find an overall winner.