You can do exaclty what you wished using Usage Controls.
Set Time Restrictions to all the time.
Within any that Time Restriction, indicate the numbers you wish them to be able to call exlusively, within your Trusted Numbers.
Then your child can only communicate with those numbers and no one else.
You can set up to 10 restricted time periods for each child on Usage Controls. It will restrict the user from calling, sending or receiving messages (including Mobile IM), from all numbers except those designated as Trusted Numbers during Time Restrictions.