Put "Find Us on Facebook," "Follow us on Twitter," and "Follow us on Instagram" links prominently on your enrollment confirmation page and/or confirmation email with a message similar to this:
"Thanks for signing up for Aloha Beach Camp...we're looking forward to a fun summer...we don't want you to miss any important information from us, so please become a fan on Facebook or follow us on Twitter so you we can share the latest information from camp with you, etc., etc., etc.")
I promise this works like a charm. I get many new fans and followers this way. The reason why this strategy's so effective is because the point of purchase, and immediately afterward, is when you'll have your customer's attention the most. They don't want to make any mistakes in the sign-up process, so they're looking to you for guidance and will almost always follow your instructions, especially at this initial point in your relationship with them.
One more thing: This idea works for any business that sells online, not just summer camps! Just be sure to tell your customers to follow you immediately after you make the sale and your fans and followers will increase nicely.