1) Cut off the brown and white tips of the cos hearts then peel away the outer leaves; use just the ones that look like they'd make good serving cups. Chop up the remaining lettuce and set out in serving bowls along with all the other salad ingredients.
2) Make the dressing by combining all ingredients in a sealable container and shaking. The kids can do that.
Let kids decide what they want in their salad cups and how much dressing they'd like.