I did find the following, all in Chelsea.

The birth records for Harry and Rudolph, and the death record for Harry on Apr 26, 1909. It lists his mothers name as Fredicka Carstenson. Age 31 y, 4 m, 13 d. Cause of death was Pulmonary Tuberculosis. The informant was Fredricka Wagner/Chelsea. Interesting in light of the following...

...there is also an index for a death record for a Frederikke J Wagner (1893; Roll 438; Page 542). If you are contacting the Archives, this may be of interest to you.

The birth register records a "male" Wagner to Fred/Fredrikka on Dec 22, 1873. Son, Fred J died Aug 30,1882(age 8y 8m, 8d) of Cerebra Spinal Meningitis. I presume he is the Isaac in the 1880 Census.

There was also a Lillian Lenore Wagner born 1882 (Roll 333, Page 258). Her marriage record for June 26, 1897 lists father as Fredk A Wagner, and mother as Jacoline F Carstenson.

Finally, marriage record for June 15, 1873. August Wagener (as written) and Frederekke Carstensen (as written). Both age 27. August's parents were Andreas and Christiane. Frederekke's parents were Jacob S, and Johanna F.

Good luck,
John