The attacker, known only as Juhanda, was found guilty of an act of terrorism committed last November when he threw a petrol bomb at Oikumene Church on Borneo island on a Sunday morning.
According to Channel News Asia, prosecutors from the East Jakarta Court said Juhanda travelled to the Protestant church in the city of Samarinda and detonated the bomb in front of the church where the youngsters, aged between two and four, had been playing.
Presiding Judge Surung Simanjuntak said there was no reason for leniency.
He said: "The defendant deliberately attacked the church, although he knew that there were many children in the churchyard."
Pray for Trinity and Anita, on the long road to recovery after a bomber attacked their church in Indonesia last year https://t.co/icZb70LZUQ pic.twitter.com/lU81DxGnnW
— Open Doors youth (@OpenDoorsyouth) July 11, 2017
Ade Intan Marbun, aged two, died from her burns, while three others were injured.
Juhanda, who wore a Muslim head dressing with IS symbols during his trial, was arrested along with four others, accused of belonging to the Jamaah Ansharut Daulah group (JAD), a local militant outfit that supports IS.
He had been previously convicted of terrorism offenses in 2011 but was released from jail in July 2014.
In a separate trial, Mr Simanjuntak also sentenced Juhanda's four co-conspirators - Supriyadi, Ahmad Dani, Rahmad, and Joko Sugito - to prison terms ranging from six to seven years.
While Indonesia has a tradition of religious tolerance and secular law, the world's biggest Muslim population has in recent years struggled with Islamic militancy and has suffered a number of extremist attacks.
A suicide bombing and gun attack claimed by IS in the capital Jakarta killed eight people in January 2016.
