The first step is to make sure your customers are even willing to help you with referrals in exchange for a discount offer. I recommend a survey to your customers asking them whether they value discount programs and what levels of participation interest them.

If you find a willing audience, one way to promote participation is to send an email with two matching coupons. Ask your customers to print the email, use one coupon on their next visit, and give the second coupon to a friend. Then offer them an additional incentive when both coupons are returned to the store. (You can use Constant Contact's "insert contact details" feature to place a unique number into each coupon by adding a coupon number field to your database.)

That being said, my experience tells me that you might be better off with a loyalty program instead of a referral program. In your case, your customers are really the kids so that's where you'll want to focus your loyalty efforts. Maybe kids who repeat get to pick a prize out of a special prize box and after a set number of repeat visits they get an extra special gift.