Would there be anything wrong with booking a fully-flex ticket so I can help my wife take the kids though T5?

Presumably i can cancel the ticket while I am airside (once the family are safely on board) and say i can no longer travel? If i book a late flight (5hrs+ later than cancelling) then nobody should be inconvenienced.

Any downsides to doing this? Presume can cancel ticket after check-in?!

Thanks