Phoenix police say 36-year-old Valerie Marie Topete is being held on suspicion of child abuse charges.
Officers were called to a pizza restaurant Tuesday night after witnesses saw Topete pour beer from a pitcher into her son's cup and the child drank some of it.
Police say Topete admitted pouring the beer in the cup because the child kept reaching for the pitcher.
The child was taken to a hospital as a precaution. He was later released to the care of his father.
