It depends, notes the International Journal of Pediatric Otorhinolaryngology, mostly on whether the kids are boys or girls.
Researchers at the University Regensburg in Germany studied the evolution of children's voices. They found that most girls showed a decline in the loudness of their screams between ages 7 and 8. For boys, the decline began between ages 8 and 9.
There was another drop in the maximum loudness level for both girls and boys at around age 10.