Child actors ?
‘Health and safety’ applies at work as well as to the general public - while acting might not be ‘real work’ 
- either way the actors need to be kept minimally safe ?
I am well past the point in my career where I am expected to deal with actors, let alone child ones.
Thank goodness.
But you’ll still have ‘vicarious’ liability to ensure your designs are at least minimally ‘safe’ for everyone ? Not just those you have a contract with.
Albeit that the actors are at arms-length from you via several levels of responsibility and contractual relationships - you could be held say 5 or 10% liable - which in a multimillion claim/settlement for an injury could still be considerable ! Check your insurances…
Okay. Yes.
The current set has a staircase and I had the balusters installed at 4" intervals. Not so much for the safety of the artists acting on the set, but more for the visual expectations from such a structure. Audience members could be made uneasy by the sight of a railing which looks unsteady.
All my sets are as safe as they need to be. I do not take the safety of the actors lightly. However, if the staircase in question was intended to look structurally unsound or dangerous for the sake of the script, then it would be built to appear that way, while still being safe for the actors. We do not have random passers by on the set. It is not built to code. The walls are 1/4 inch lauan on 1x3 frames.
We build to represent life, not imitate it. If I need a staircase with no railings, or minimalist railings, or representative railings … Or the ruins of a railing… All of these are kosher.
But yes. Safety is important.