Assuming you can't get explicit consent, illegal.
California Penal Code section 632(a):
632. (a) Every person who, intentionally and without the consent of
all parties to a confidential communication, by means of any
electronic amplifying or recording device, eavesdrops upon or records
the confidential communication, whether the communication is carried
on among the parties in the presence of one another or by means of a
telegraph, telephone, or other device, except a radio, shall be
punished by a fine not exceeding two thousand five hundred dollars
($2,500), or imprisonment in the county jail not exceeding one year,
or in the state prison, or by both that fine and imprisonment...
California's Supreme Court recently reiterated the meaning of a
"confidential" communication in a case involving the broadcast of the
aftermath of an automobile accident. The court stated that a
conversation is confidential for purposes of the eavesdropping statute
"if the circumstances objectively indicate that any participant
reasonably expects and desires that the conversation itself will not
be directly overheard by a nonparticipant or recorded by any person,
participant or nonparticipant."
People v. Gibbons, 263 Cal. Rptr. 905 (Ct. App. 1989) established that
section 632 also applies to videotape, even if it does not record
Note that this applies only to recordings in places where there is a
reasonable expectation of privacy. Though it doesn't seem likely with
a young child, if any of this roughness happens in a public place,
there is no reasonable expectation of privacy, so such recordings are
legal. If it happens on a regular basis, you may be able to engineer
an outing and hire a private investigator to follow and record at a
Here's a link that provides some tips about recording surreptitiously
in places where there is not a reasonable expectation of privacy:
While the baby is residing in your house, you may wish to consider
hiring a nanny to keep an eye on her while you are unable to. After
that, I'm afraid you're in more turbulent waters.
Search terms: california video recording "expectation of privacy"