Recent secrets I've been asked to keep and not told dh (their dad) include health fears (GP sorted) and love life dramas.
A secret dh has been asked to keep (but told me anyway) is the purchase of an Xbox game I've banned that is played while I'm out. 
So I'm torn. I would be gutted if dh hid our dc's health concerns or emotional turmoil from me. But at the same time I want my teens to trust me to be able to tell me their stuff and not tell their dad. Double standards, no?
So what's your line on this? Do you tell your co-parent what's going on in their lives in confidence or keep mum?