Please do not feed the weirdo - R. L. Stine

Robby and his sister Karla beg their parents to take them to a big
carnival that has opened on the other side of town. When they arrive,
the two kids are delighted by the rides, the sideshow, the
interesting displays, and the great food booths.
They wander away from their parents and find themselves at a less-trafficked
area at the back of the carnival. Inside a large penned-in
area, they see a dejected-looking boy about their age sitting
on the grass. A sign on the tall metal fence reads: PLEASE DO
NOT FEED THE WEIRDO.
The kids are reluctant to disobey the sign, but the boy seems really
nice. Karla hands him her cone through the fence. He thanks her
very politely. He eats the ice cream, delicately at first, then
ravenously, noisily - and as Robby and Karla stare in horror, he
transforms into a raging, hairy beast.