A babysitter who sexually abused two seven-year-old boys in Perth has been sentenced to five-and-a-half years in prison and is likely to be deported to Sri Lanka after serving his term.

Dayanthan Stanley Mohanraj, 19, was convicted after a trial of four counts of sexually penetrating a child in 2016, when he was babysitting on a casual basis.

Stanley Mohanraj showed one boy pornography and later sexually abused the child in a public toilet, then threatened to kill him if he told his parents what happened.

He separately abused the same boy in a playroom and on another day attacked both children while they were playing, telling them to keep it a secret, the West Australian District Court heard on Tuesday.

Judge Vicki Stewart said Stanley Mohanraj lacked remorse and was unwilling to accept responsibility for his offending against two vulnerable children, which involved a serious breach of trust.

"You took advantage of these two young boys. This is abuse of the most serious kind," she said.

"You were in a position of authority. You were trusted to look after these children.

"You made comments to each boy to secure their silence."

Stanley Mohanraj came to Australia from Sri Lanka in 2012, but his family's refugee status has been rejected and they are in the process of appealing in the Federal Court.

He must serve at least three-and-a-half years behind bars before he can be eligible for parole and will then likely be deported.