Why do children prefer sweeter foods?
@Colombo explains one reason that I think is obvious. However, there has been some research done on this.

One other reason is because it would provide an evolutionary advantage in environments where calories are scarce. Also, sugar actually acts like a pain reliever. Studies show that giving sugar to babies and children during surgery act like a pain reliever. 

The sweet tooth could be controlled by hormones secreted from the growing bones. Some common hormones like insulin also affect sensory centers in the brain. This explains why the sweet tooth goes away as an adult.

Reference

NPR -http://www.npr.org/sections/thesalt/2011/09/26/140753048/kids-sugar-cravings-might-be-biological