BOSTON (CBS) – A three-year-old boy was killed and his four-year-old sister is badly hurt after they were struck by a van in South Boston Wednesday afternoon.

A woman was pushing the children in a stroller when they were struck by the van on a sidewalk. The crash happened at the intersection of L Street and East 6th Street at about 3:15 pm.

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Police say a car hit the van, causing it to go up onto the sidewalk.

A van drove onto a South Boston sidewalk, striking two children (WBZ-TV)

“Our heart goes out to the parents obviously,” Boston Police Commissioner William Evans said. “Terrible tragedy here. It’s everybody’s worst nightmare, walking along the street, and we have an accident where we have two young babies hit.”

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The woman pushing the children was not hurt. It is unclear what her relationship is to the children.

Susan Devlin heard the collision and called 911. “It was just seeing the child and the carriage. I think it hit the carriage the carriage went up,” Devlin said. “Horrendous, it was horrendous and seeing the sister screaming for the little child it was a horrible scene.”

She’s says people rushed over to help. “An angel walking down the street who gave CPR and held them and kept them calm,” she explained.

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The drivers of both cars stayed at the scene. The intersection was closed while police investigate.