Several children can only be linked in a parent profile if they go to the same school. If your children go to different schools, you have to register with two Sdui accounts. This means that you will receive an activation code from both schools and register with these codes.
In order not to have to constantly switch profiles by logging out and logging in again, you can manage several accounts centrally in Sdui. Instructions for setting up the cross-school account link can be found here: