who helped the child injured by mr Hyde?what did he demand?

Utterson did. He demanded a cheque be written to the girl.

Please disregard the above,

One night, at three in the morning, Mr. Enfield was walking through town when he saw a disfigured man whom he described as "a Juggernaut," powering through the street maliciously trample an eight-year old girl who was out to fetch a doctor. After apprehending the man, Enfield, the doctor, and the family of the girl decided that, instead of sending for the police, they would blackmail the man to give one hundred pounds to the girl's family.