Reading between the lines of the arrest report, that's probably why Nick was willing to talk to the cops, specifically about who lived in the master bedroom. All the listed drugs were found in the master bedroom and master bathroom safes, none listed as being on anyone's person. It seems like he stonewalled them on other questions, which sounds like he was trying to convince them to let April off the hook.
Cops naturally arrested her at the scene to make sure, and the stories were probably close enough to warrant releasing her. If the search warrant was based around a child endangerment report it's unlikely to mention April, so unless they can prove a visiting guest had access to the safe, the possession charge looks extremely thin.
Speaking of which, how did the cops get access to the safes? Would a child protection search warrant cover that, and if not could the cops on the scene compel Nick to open them? It seems unlikely he'd comply given his attitude towards the door and the warrant.