June saw a surge in attacks in Cabo Delgado province, where jihadists launched a bloody insurgency in 2017, sparking a regional military mission last year that had restored a sense of security.
At least 53 people were killed in several districts, forcing more than 50,000 adults and children to flee their homes, the charity Save the Children said.
"This has been the worst month for families and children in Cabo Delgado in a year," Brechtje van Lith, its country director for Mozambique, said in a statement
She said people fleeing the violence, which has displaced more than 700,000 so far, were running out of options for shelter.
"And yet this is not the first time they are going through this -- many are experiencing violence for the umpteenth time with no end in sight," she said.
The charity sounded the alarm about mental health.
"Many of them have lost loved ones or witnessed horrors that no child or adult should ever need to see," she said.
Nearly 4,100 people have been killed in Mozambique since 2017, according to the conflict watchdog ACLED.
Some 3,100 troops from several African countries deployed in Cabo Delgado last June and retook control over much of the territory.