Chances are they are children who died in infancy
it is not always easy to find them
birth/death certificates (for states that had them 1900-1910)
Family Bible records
family letters and journals
@Lisa Whittle Talbert depends on which children youre referring to, if youre looling for the ones that have passed, then @Dennis J Yancey has given great info for that. If youre searching for the children who are living, I would check for birth records using parents names, check findagrave and will/probate records for the parents to see children listed. I would also i would do a surname search for those same census years for the location where the parents lived and nearby locations... if you post a link to the census record here, along with the names of the parents and any other info, there are volunteers who may be willing to help. I hope this was helpful!