As Fathers across Australia awake on Sunday to appreciative offspring bearing gifts, they could spare a thought for the child grooms of Nepal who can be as young as seven.

Child marriage is mostly an issue for young girls but a new report by CARE Australia says it can also be a problem for young boys.

That can lead to psychological trauma and high rates of dropping out of school to support their new families, it found.

In most places where child marriage is common, older men wed younger girls.

But in parts of western Nepal, boys and girls are often forced to marry each other.

CARE said these pre-teen children went through wedding ceremonies, then lived apart for a few years. When boys reached their early teens, the couples moved in together with the expectation of starting a family.

CARE country director in Nepal Lex Kassenberg said this was hardest to take for poor families who could least afford to take risks.

"Wait too long, they fear, and their child will be stigmatised and unable to marry," he said in a statement.

The report says 18 is the legal marriage age in Nepal but that's largely ignored for both girls and boys in western Nepal. In one district 26 per cent of girls were married by 14 and for boys the figure was 12 per cent.

The report found former child grooms, many trapped in poverty, were emerging as key allies in the movement to end early marriage.

"They know what its like to be trapped between boyhood and fatherhood," Mr Kassenberg said.