Hi Scott,
Came across your post while working at the FHC. I did some digging and found 3 sons born to Martin Seedorf and Betha (Beta) Voller Kocher.
Sons are as follows:
Johann Eberhard Seedorf b. 2 Apr 1868 Lesum, Hannover, Germany (Parish, Hamburg)
Johann Hinrich Seedorf b. 19 Feb 1870 Lesum, Hannover, Germany (Parish, Hamburg)
Martin Diedrich Seedorf b. 1 Oct 1871 Lesum, Hannover, Germany (Parish, Hamburg)
Here's a link to that info:
https://www.familysearch.org/search/record/results?surname=Seedorf&father_givenname=Martin%20&father_surname=Seedorf&film_number=953308&count=20&offset=0These births can all be found on FamilySearch film number 953308. Not sure if you found these or not. There is approx 7 years between these children and Anna. Lesum is about 13 km from Buschhausen. So not terribly far.
I did find Betha's baptisms record based on the info you provided. Her baptism record indicates that she was born Unehelich (illegitimate) so Betha (often Beta in records) was given her Mother's maiden name of Voller. Not sure if that has anything to do with not locating Anna's baptism/birth record. I'll dig a little more while I'm here today and let you know if I find anything.
Debby