Have you found them in the census records? William apparently died between 1860-1870, and Euseba remarried a WP Foster (see 1870 census, Winn Parish).
Besides the two Brock children (Amanda 16 & Robert 14), there are two older Foster children (Margaret 18 & William 16). There are also two young Foster children, Angeline (4) and George (2). I would work on the assumption that the two youngest children are likely Euseba's, which would mean that William presumably died by 1865.