From what I understand, it is not an offence to have pictures of yourself (Of course...) or family members naked as a child, providing it is innocent, like bath time for example. Even if they were to take you to court over having pictures of naked family members in such situations, I doubt it would hold up in court.
But of course, as soon as there is any evidence of any sexual nature in the pictures, then it would be classed as "possession of child pornography". I think for someone to be charged with sexual assault of a child, then he/she would either need to be witnessed doing the crime, or have damning evidence put against them, like a video or a picture of him committing the act with his/her face in view.