So we drew 2 people talking and added the conversation that would happen at home. Then we looked at how it would look in a story. (Our first draft in our books we used stick figures)
We need to remember that speaking does happen in a vacuum. In other words the people are doing something, cooking dinner, playing on the PlayStation...
Each person who speaks what they say starts on a new line.
If only two people are speaking that the third or consecutive lines do not need the tag line 'said Mum.' only the first two need it.