Talk about a sad time in their life, what made it sad, how did they feel.
Draw a picture and writing linked to this, a little worried how sad they will get and what they might say any ideas for leading them in a good direction they are year 1.

i think if you model something for them along the lines of 'I was sad when my friend was poorly' or 'I was sad when I ripped my favourite jeans'

you will be setting the scene and at this age they generally follow suit, so i would be surprised if they ended up offering situations that are too deep or too sad.

Dont confuse them with good and evil though as they may think its wrong to get sad and children need to know its ok to feel their emotions