If you don't have aprons for the children give them an old adult-size T-shirt to wear over their clothes.
Let the children choose a color for your picnic. Then try to incorporate that color into your picnic plans as much as possible. For example, if they choose green, you might wear green clothes, sit on the green grass, use a green tablecloth, eat green grapes, and celery with cream cheese mixed with green food coloring, play with a green ball, or take a walk and look for green things.
How can I make an easy costume?