I used the abandoned house for this one although you can probably use any of the small houses for this. Take out all walls and items so there is only a bathroom and one big room. Cleanup and paint so it's decent but not necessarily fancy. Buy and place these items:
Mantis bed
bookcase Kall
Venne Horizontal Bookcase
Standard Desk
Swivel Chair
Tub, Toilet, and Sink in bathroom
2-3 Bears, 4 Balls, 4 Bunny Rabbits (big and small)
The toys are to get rid of the other buyers who would outbid him but don't like kid stuff