When they already have Kids
When both the couple has kids or even if one of them already got kids, it is important that they include their kids at the wedding. If the bride has a daughter, she should include her as part of the entourage according to her age. She could be part of her flower girl, bridesmaids or even her maid of honor. The same goes if she has a boy. And the same goes for the groom.
Including the children to the wedding will also create a much better atmosphere with the guests and a much better relationship with the kids. Not including them may even signal a wrong impression towards them, which may not lead to a good start. To keep them happier, do not forget to give them their necessary gifts such as their flower girl jewelry or bridesmaid jewelry so that they will feel that they are truly part of the wedding party.