A 3-year-old girl and her nanny drowned yesterday after they slipped in the deep end of a backyard pool on Long Island, authorities said.

The horror unfolded in Lawrence at about 2 p.m., when the nanny, Lorena Canales-Mejia, 32, of Inwood, and the little girl, Naama Markovits, were apparently splashing around in the shallow end, sources said.

“She may have had the 3-year-old in her arms and ventured too close to the deep end and was unable to regain her footing,” said Nassau County Police spokesman Lt. Detective Kevin Smith.

Cops said a 5-year-old boy at the home spotted the pair and alerted a painter working on the house.

He pulled them out and attempted to resuscitate them, but they were unresponsive.

The toddler was pronounced dead at a hospital; the nanny died several hours later.