The newly born lamb or calf is automatically placed in the same mob that the mother is in. The idea behind this is that after a few weeks you separate the children from the mothers with the menu "weaning". At that point, you place the lambs in a different mob.
You can place the animals in a separate mob immediately at birth. Go to Settings> Birth Menu Options> Place lamb in other mob than mother = ON. From that moment on, the app asks you for each newly born animal in which group you want to place it.