South Brunswick police tell The Home News Tribune the family was returning home after stopping at a coffee shop when the 8-year-old began having trouble breathing near Route 27 Sunday.
Detective Sgt. Jim Ryan says the boy went into cardiac arrest when police arrived and an emergency injection failed to revive him.
The child was taken to University Medical Center of Princeton, where he was pronounced dead.
Ryan says the boy was allergic to milk and it's not clear if the 8-year-old drank it by mistake.
The family's name has not been released.
Information from: Home News Tribune (East Brunswick, N.J.) , http://www.mycentraljersey.com