In a competitive game, the lowest score wins. In a cooperative game, don't keep score. Focus on the fact that everybody hits the target.
Young children, under age 5, want to play and hit the target. They are not concerned about rules and about practice. It is more fun to accept their "way of playing" than to force them to accept rules. Rules are followed as long as they are able to do what the adults and older children do. But since preschoolers lack skill, it can be frustrating to "play by the rules." So they bend them.
After age 5 or 6, the rules are accepted rigidly. They will even add rules that they make up. They will have a problem playing with a younger child who does not play by the rules. it is equally hard for this age to understand little kids. Each see only their point of view.
1. Preschoolers learn the sequence of going from target 1 to 2 to 3, etc.
2. The targets can spell something. For example, they must spell J-A-N-E by going to targets in the correct order.
3. It might be fun to write down the rules together. The children can dictate them to you and you can write them down.
4. Make score cards that they write their name on.
5. Use addition skills when they add up the scores.
6. Have the kids make and decorate a new set of targets. They can be boxes, hoops, pictures... They could even throw at themselves. The Frisbees are foam.