Talking to my child about my mental health
Mind have some information on their website around talking to your child about your mental health problem and parenting with a mental health problem.
Talking to children about illness
The British Psychological Society have published some guidance that helps people to talk to children about illness, taking into consideration their developmental stage.
Telling a child someone is dying
Marie Curie have information on their website that supports people who need to tell their child that someone is dying.