It may be helpful to know the circumstances under which the children were placed with other family members &/or an orphanage:
1. Did Theodore &/or Maggie die?
2. If so, do you know when?
Theodore, Maggie and Eva were enumerated in the 1905 Iowa census in River Sioux, Sherman Township, Monona County.
If you know the circumstances of the parents' deaths, have you already tried to procure any will or probate documents that might be on record at the county courthouse? If that isn't fruitful, it may be helpful to procure all four grandparents' will/probate records in case any of them make reference to Theodore/Maggie &/or their children.
In order to strengthen or eliminate the theory that Rufus HELDT was later Don MANTEL/MANTAL, it would be very helpful to compare Rufus's exact birthdate to that of any MANTEL//variant// you can locate. The only Donald MANTEL//variant// I've been able to find was in the Social Security Death Index, with a birth date of 7 Dec 1913, and the SSDI and other records indicating residence in Ohio.
Can you list the other siblings' names? There may be a reference to Eva &/or Rufus in various records or newspaper items.