Los adultos compran este calendario para los niños porque tienen chocolates y son deliciosos.
Tienen is plural while calendario is singular. So who have the chocolates? The adults or the children? Neither makes sense.
This sentence is funny in a way because you could read as the children have chocolates and the children are delicious. I guess the adults buy the calendar to lull the children, to capture them and eat them. Yum, delicious children:-)