My thoughts are with those people. Believe Unwell Raptor and Dallas Dude have it right, friendly fire supported by US awacs type aircraft. Peru's main newspaper, El Comercio, say Iquitos tower confirmed that the float Cessna had filed a flight plan via radio on departing Islandia (which if I recall correctly is a lumber mill downstream from Iquitos). The woman and infant apparently died from rounds fired by the FAP aircraft and the pilot, Kevin Donaldson, was wounded badly in the right leg - exposed fracture.
Although the area is now rife with drug smuggling aircraft, Dallas Dude will remember there are dozens of missionary aircraft on floats operating there as well. I would imagine missionary pilots would be exceptionally alert to military aircraft and totally up to date regarding signalling, manoeuvering, radio procedures for identifying themselves correctly. The other woman on the aircraft said the attack came without any warning. Donaldson has to date given no statement.
None of the reports I've read say what time the attack occurred, but you don't fly around there on floats at night, full stop.