Thank you all for your comments. We have decided to do some test shoots to test out the best method for us.
Thank you for the advice. We are planning on pre recording the track to have them sing to on set like you said. I am intrigued by actors singing live on set but, technicalities aside, I feel this may restrict our angles and put a lot of strain on the actors. Also I feel recording sound while having playback may be a big issue.
I'm hoping to have a good couple of days rehearsal before recording the tracks so they understand the emotional levels involved in those moments before laying down their vocals.