The man, 27, struck the elderly Good Samaritan multiple times and broke her arm before he was apprehended by police.
A statement by Duisburg's police department said the woman attempted to block the man from reaching the child by putting herself in between them. The 27-year-old man hit the woman several times.
He then tried to steal a wallet from the woman's hand, "twisting her arm so forcefully that it broke," police said.
The man fled after other bystanders came to the woman's aid. The 85-year-old woman sustained a broken bone and numerous cuts and bruises. She was taken to a hospital.
Police immediately launched a search effort to try and track down the man. He was arrested and is being arraigned.