The best way to know if you or the other parent could be affected is to assume you WOULD be affected and then go from there. There is no bright line law or case law currently in California which seals a parent's fate as to marijuana in general right now.
Attorney herein researches these issues as they apply to California Family Law, and makes best efforts to remain current on new cases or decisions. Attorney is well aware of the long running case from Butte County that then seeming moved to another county just north of Butte, with varying detriment (depending on what one considers detrimental...) In any event, any type of drug, even if it's a prescription, can be cause for concern in a divorce case.
We all know that alcohol use can be a concern, but it's legal to buy it, right? It's legal to obtain many drugs straight off the shelf at the store. When courts have to consider whether drug usage is going to affect custody, it will depend on what evidence is available, how good the attorney is at setting up the defense or attack of the issue, what the exact facts are and are they verifiable, and how; essentially, all of the foundation requirements for evidentiary hearing, and all of the foundation requirements for supporting evidence should be known; further, the lack of such evidence and the preclusion possibilities, as with all evidence, will come into play.
Trying cases with drugs involved should be left to attorneys who handle trials, because the proof and litigation aspects can be done with more precision. Many clients don't want to spend the funds to engage litigators to gain appropriate custody because they do not understand the time involved to make the defense, or the admissibility for proper charging evidence.
In family law we usually see many clients that just believe all they have to do is say something, and that makes it true. That is not the case necessarily. Conversely, clients often believe they can just say something is not true, and that will prove their case. That too is not necessarily true, because if it was, then there would be no NEED for any attorneys at all, because clients could prove everything themselves?
Obviously, clients, at least most of them, are not able to do that because they did not go to law school and then work for 20+ years at honing their skill. We have to deal with inaccurate facts, misleading statements, the social media nightmare, Facebook (often the absolute worst), and tons of Internet postings, admissions, accusations, texts, blogging, examples: Lively, Instagram, Vine, Snapchat, Kiks Messenger, WhatsApp, GroupMe,tumblr, Twitter,Musical.ly, You Now: Broadcast,Chat,and Watch Live Video, Burn Note,Whisper, Yik Yak, Omegle+Tinder (dangerous/or not appropriate for kids).
New apps come out all the time and they will never stop. These things can be worse than drugs in some ways as unknown people can take advantage of kids, teens, etc. A parent would never even know because most parents don't really know what kids are doing with their phones.
Personally, we would not give kids a phone, computer or any electronic device which was NOT monitored, it is very easy to obtain software to do this!