MRC des Collines police say officers responded at 5:42 p.m. ET to a home at 82 chemin Éméralda in L'Ange-Gardien, a suburb of Gatineau.
They found a boy who had been struck on the head by a tree while playing behind the family home.
He was taken to nearby Buckingham hospital before he was transferred to CHEO.
The boy was listed in critical condition when he arrived, police said.