The children had seen the ball at a nearby deserted compound and mistaking it for an ice cream ball picked it up and brought it home.
A police dog squad, explosive experts and others are inspecting the SUV in which the explosives were stored.
The driver of the vehicle has been traced and sketches of the two people are being prepared.
CPI alleged that RSS workers were behind the attack.
Tuesday's blast was one of the deadliest to hit a region under the control of Turkish-backed forces.
He was in jail for hoax bomb calls in 2018 to Kempagowda International Airport and Krantiveera Sangolli Rayanna railway station.
A special court here on Friday awarded death penalty to four convicts for the 2008 Jaipur serial bomb blasts that had left 71 dead and 185 injured.
At least two persons were injured when unidentified miscreants triggered bomb blasts at five locations in Polasra town in Odisha's Ganjam district.
The arrested were Panthakkal branch secretary K P Biju, 39, and his aide Rinoj, 27, who was tasked with hurling the bomb at the CPM local-level leader.
The accused, said to be a jilted lover, called the airport authorities about bomb threats to Indigo and Truejet flights at Rajiv Gandhi International Airport in Hyderabad.