Police have taken a nine-year-old boy into custody over an acid attack that left two toddlers with burns to their faces near Melbourne's busy Chapel Street today.

The boy will be questioned by detectives this afternoon after a powerful liquid, believed to be a detergent used for cleaning tiles, was thrown onto the pram from a roof-top carpark in the South Yarra shopping district.

Police said the mother was pushing her five-month-old boy and two-year-old girl in a pram along Bray Street, which runs parallel to Chapel Street, about 12.30pm when the liquid drenched their pram and hit the children’s faces.

The distraught woman brought her two screaming toddlers into Caffe Sienna Ristorante on Chapel Street, where police and paramedics were called.