If you want to add a child to an existing family tree, it's easiest to establish the kid's parents' relationship first. Given one of the parents is called "Linda", you'll click on Linda's name on the tree, and an info box on the left side of your screen shows you all the personal data you have added for her (such as life dates).
Below these information is a button with "Add partner/ex" that allows you to add the person Linda should have a kid with; we'll assume the second parent's name is Ahmed in this example.
After you have added Ahmed (of course you can skip the first step if both parents are already a couple (or ex-couple), you click on the button "Add child" that appears below the button "Add partner/ex". If you click on "Add child", you'll see the options "Add child with Ahmed", "Add child with new partner" and "Add child without partner". In our case, you'll of course select "Add child with Ahmed" and enter the child's name, life dates and other information (i. e. "female, Layla Jones-Aciman, born 2015 in Birminghan") you want displayed.
If you want to add more than one child per couple, you can either go the same route of "Add child" -> "Add child with" or just click on the child already existing and select the option "Add brother/sister".
Hope this helps!
