A spokesman for the Angolan Christian Churches Council, Jaime Paulo, said the children were killed on Tuesday, during an attack on the town of Beu in the north of the country, close to the border with the Democratic Republic of Congo.
"Christians are the object of false accusations, because they take a neutral position in armed conflicts," Mr Paulo said.
He added that Unita had killed more than 80 Christians so far this year on suspicion of providing information to the army.
Account not confirmed
The churchman's account has not been independently substantiated, and Unita has not responded to the allegation.
The statement also said that Unita had recently captured two diamond mines in the Lunda Norte province.
There has been no word from the Angolan government on the Unita statement.
The Angolan civil war has been running for nearly 30 years.
It resumed in 1998 after a fragile four-year-old peace accord collapsed.
Since November 1999, a successful government offensive - with the help of Namibian government forces - has resulted in the capture of many important rebel-held towns, including Jamba, where Unita's headquarters was once located.