Adding children to your private Kinderloop is very easy, simply click the children settings button and then click Add Child, you can choose whether to invite family at this stage or later when ready.
If you are in the app, login and navigate to your web settings > children and add each child you want to document on.
Remember, your Kinderloop is private to you and nothing is seen by family until you invite them.
When you create a post, tag the child/ren in that post which will alert the linked/invited family members that there has been an update for their child.