This is a horrible event, and the horror goes beyond the targeting of people on their way to worship. The horror is that a family, including a 9 year old child, are the ones responsible for the horror.
For Americans, I suspect at least a part of the horror will be (perhaps should be) that the attack by an as-yet unnamed family on three Christian churches occurs on Mother's Day. While Indonesians celebrate Mother's Day in December, so the date isn't an intentional horror, Americans are wont to see every world event through our own lens. There may be several last-minute additions to church homilies mentioning the significance of the day in making their congregations more aware (read: afraid) of international terrorism.
But let's keep this in Indonesia, where the horror is enough without the symbolism. A family consisting of parents, sons in their late teens, and daughters ages 9 and 12, divided into three groups targeting three different churches, one Catholic, one Pentecostal, and a third that sounds like a generic Protestant congregation. That the family was Muslim, or at least saw themselves as Muslim (because many Muslims would argue that by doing such a thing the family gave in to apostasy), is probable since the Islamic State claims responsibility. And the targeting of a populous public space by suicide bombing is a well-known IS tactic, while researchers have noted the rise of intolerance by majority Muslims against other religions.
But the horror, as I've suggested, lies in this otherwise bland statement in the BBC's report: Women "have become increasingly active in terrorist cells in Indonesia but this would be the first time children have been used."
Not discounting the mid-millenium Christian Children's Crusade, some of whose leaders may have been younger than their early teens, does the active use of children in violent anonymous attacks suggest a surety in the rightness of the groups' cause? Or a sense of desperation in the leadership encouraging them?