SSE Version
Safehouse Children's Room is an option for Safehouse Room B. You can buy the receipt from Bits and Pieces in Solitude.
Children's Room B
If you buy the "Safehouse Kids Furniture Receipt (B)" (3040 Gold), it will be automatically added to the Household Planner in the Master Bedroom. Here you can set up the room to contain 6 beds, 6 chests, dresser, shelf, practice dummy, rocking horse, 1 table, deluxe Dragonsreach dollhouse, toy box and assorted decor and children's toys for your adoptive kids.