The children were the subject of an Amber Alert Sunday.
Their father does not have custody. Police say he held a 13-year-old babysitter at knifepoint.
Moline police say a man believed to have been their noncustodial father, 36-year-old Delfino Sierra took the children early Sunday morning while a 13-year-old nephew was baby-sitting them.
The missing children were identified as 1-year-old Judith G. Sierra and her 6-month-old brother, Delfino Sierra Jr.
Police say Delfino Sierra Sr. is not married to the children's mother and is also known by the aliases Jesus M. Carrion and Jesus M. Melendez.
The Associated Press contributed to this report.