A "childhood secret" is something that happened or is known only by you during your younger years. It is something you may have kept hidden from others for various reasons. Full definition
He had the look of a teenager watching his girlfriend talk to his dad, sure that at any moment a deadly and embarrassing childhood secret would be communicated. (politics.co.uk)
The original film, which was later officially revealed to be the most profitable film of 2011, will again follow the unlucky Lambert family as they're lives are still haunted by the previous events and who need to look to the past to discover the mysterious childhood secret that explains their dangerous connection to the spirit world. (thehollywoodnews.com)
Anyways, we move from one awkward scenario to an even more awkward situation where Brad is finding out about all of Kate's deep dark childhood secrets, and at the same time having some real hilarious conversations with Gram - Gram and Marilyn. (gmanreviews.com)