The children died in the New Baghdad neighbourhood in the southeast section of the city, police captain Sabah Hamid al-Fartusi said on Friday. Fighters frequently use hidden roadside bombs against US and Iraqi army convoys.
Three men killed an Iraqi army officer, Major Mahmud Hasan al-Yassiri, late on Thursday in the southern city of Basra, captain Firas al-Timimi of the Iraqi army said.
In the Shia holy city of Najaf, four civilians were injured by a bomb that exploded near a bus station, local police captain Qussai al-Jazairi said.
US marine killed
A US marine was killed on Wednesday in a vehicle accident during combat operations in Falluja, 65km west of Baghdad, the military said on Friday in a statement.
Also on Friday, police in Kirkuk, about 290km north of Baghdad, said one driver was killed in an attack that set several Turkish oil tankers ablaze the previous night. Six others were wounded.
The violence came a day after Shia Arab Ibrahim al-Jafari, one of Saddam Hussein's most implacable enemies, was formally chosen as interim prime minister.
Al-Jafari was selected on Thursday by a three-member President's Council led by President Jalal Talabani, a Kurdish leader.
The process was delayed for weeks as the country's politicians tried to settle on which Sunni Arab lawmaker would become parliament speaker.
Hajim al-Hassani was eventually named as parliament speaker - an effort to reach out to the Sunnis.
Al-Jafari has a month to name his Cabinet, but the National Assembly's primary task - drafting a permanent constitution by 15 August - has not been addressed.
PM al-Jafari (R) with President
Talabani are the new leaders
Sunni Arabs have only 17 seats in parliament, largely because many boycotted the 30 January elections or stayed at home for fear of attacks at the polls. Shias have 140 of the 275 seats in the National Assembly, while Kurds have the second largest bloc with 75 seats.
In writing the constitution, lawmakers will have to address the diverse demands of the country's different religious and ethnic populations, including the role of religion in the government and Kurdish demands for control of the oil-rich area of Kirkuk.
If the constitution is approved in an October referendum, elections for a permanent government will have to be held in December.