A woman who was at the party for her 10-year-old grandson said the shooters, "three dudes," appeared to be firing at people who ran toward the party late Tuesday afternoon on a porch on Simon Bolivar Avenue, The (News Orleans) Times-Picayune reported.
At least three people, including the birthday boy, were wounded.
Police said the woman was about three blocks away at the wheel of her Mazda when she was hit. WWL-TV reported she was driving to the party.
Esther Anderson said she was next to her granddaughter when the 5-year-old was hit.
"I just buried my son, now my grandbaby. ... I can't take too much of this," Anderson said.
Mayor Mitch Landrieu, who came to the shooting scene, said it was the work of "thugs."
There were several other shootings in the city Tuesday, including one that killed a 58-year-old man as he sat on the steps of his house. Police arrested two teenagers, one 15 and the other, the alleged triggerman, 13, WWL reported.