You can’t attach a child to a parent until that parent has been saved. Saving the parent creates the Salsesforce ID that is used to associate the child, so you will have to save the parent first. You could try to do everything with one button, but I think you would have better luck with a button that saves the parent record and then, once saved, allows you to create children. On your child model, you would add a condition that the parent ID is equal to the ID of the record in your parent model. Make sure your parent model is set to only have one row. Your newly created parent should become row 0 and be used as the row for your child model condition. Then, when you create a new child row, it should auto populate the parent field with the ID of the parent record.
