what does \"treated as a child of the family\" mean?

My husband's son (my step-son) has never lived with us, although we have taken him on holiday with us occasionally and he has visited our home once or twice. He does not have regular contact with his father.

Do I need to include him in point 5 of the introduction section of the divorce petition form D8("There are no children of the family living except...") and do I need to provide his details in the Statement of Arrangements for Children (form D8A)?

The child in question is 16 and still at school full time.

No, You'd only need to put him if he'd lived with you on regular basis.

If he's 16 he's not a child.

You need to list living 'children of the family' at any age who lived with you and were brought up as though they were your own children. As your step son never lived as family Q5 doesn't apply.

However, when there are children of the family their dob needs to be provided for under 18s and where the child is over 16 but under 18 it should be stated if they attend school,college, training or are working full time. For 'children' over 18 you just state they are over 18.

See D8 Notes available from HM Court Services website.