Two young girls who went missing from their family's campsite overnight have been found safe and "in good health," Ontario Provincial Police say.

The girls, who are cousins, went missing from the family's campsite near Aird Lake, on Airds Lake Road, about an hour's drive southwest of Calabogie at about 9 p.m. ET Monday.

Phil Ripley says his family has been visiting the area since he was a child. (CBC)

Elya Munro, 9, from Barrie, Ont., and Ember Greatrix, 11, from North Frontenac Township, Ont, were found after they heard and responded to an OPP search team about a kilometre or two into the bush, said OPP Const. Beth Ethier.

The girls disappeared from the site, which is part of a 243-hectare cattle ranch, following a game of hide and seek with relatives.

“We’re just so thankful for everybody that’s been here to help and so relieved they found her,” said Munro's aunt, Lynn Ripley.

“I will never begrudge paying taxes again," said Greatrix's grandfather, Phil Ripley.

"When you see what kind of response we get, your heart just warms up.”

Fears of hypothermia

The police search began at about 10 p.m. Monday and continued overnight. A helicopter was dispatched early Tuesday morning, along with additional officers and canine units.

The girls were wearing flannel pyjamas and running shoes, and it was raining in the area overnight.

Renfrew County paramedic chief Mike Nolan said before the girls were found that paramedics were standing by at the staging area for the search.

"We had a significant thunderstorm come through the area last night, so our concerns range from exposure, them being cold, potentially hypothermia," Nolan told CBC News.

"Our goal here is to locate the children, get them immediate medical care with the paramedics on site and then reunite them with their family."

Relatives said the family had visited the campsite for decades.

“In the '50s my father, my two brothers and I got lost in the same place. We overnighted in the bush and wound up at the Calabogie bridge," said Phil Ripley.

"I’m 73 and that was the most horrifying night of my life … that’s all I could think of. I had my father and brothers. They had nobody. We didn’t even know if they were together.”