We found out he had a brother Carlo born in Cis July 27, 1895.
I saw that birth record too, But I also believe that the 1900 census is correct for Domenico Mengoni's family:
1900 United States Federal Census
Name:
Dominic Mengony[Dominic Mengoni]
Home in 1900:
Northside, Sweetwater, Wyoming [Bitter Creek, Sweetwater, Wyoming]
Age: 39
Birth Date:
Oct 1860
Birthplace: Austria
Race: White
Gender: Male
Immigration Year:
1887
Relationship to head-of-house: Head
Father's Birthplace: Austria
Mother's Birthplace: Austria
Spouse's Name: Paulina Mengony
Marriage year: 1894
Marital Status: Married
Years married: 6
Occupation: View on Image
Neighbors: View others on page
Household Members:
Name Age
Dominic Mengony 39 head, born in Austria on
October 1860, married since 6 years, immigr. year
1887, naturalized
Paulina Mengony 37 wife, born in Austria on
December 1862, married since 6 years, mother of three children all living, immigr. year
1894
Charles Mengony 4, son, born on July 1895 in Wyoming
Rentina Mengony 2, daughter, born on October 1897 in Wyoming
Virginia Mengony 1, daughter, born on January 1898 in Wyoming
I would advice you to contact your nearest Family History Center and ask them to rent the microfilm of the church acts of the parish church of Cis:
Subjects
Italy, Trento, Cis - Church records
Film Notes
Note
Location
Film
Battesimi 1633-1923 Matrimoni 1638-1808
FHL INTL
Film 1388652 Items 26-31
Matrimoni 1822-1923 Morti 1664-1923
FHL INTL Film 1388653 Items 1-6
you'll be able to see the original church baptism acts...
It's also possible that they decided to baptize the children once they came back to Cis, because from the 1900 census looks like that the children were really born in the U.S.
Charles is Carlo and Rentina and abbreviation of Fiorentina...
Or, ask to the proper office of the Comune of Cis the birth acts for the children...
Laura