No info is useless! It sometimes just needs backup elsewhere. I tried your up/ link but currently offline. Regards your Jan Hendrik Kapp - 4 of the children had Jan or Johannes in their first names (two Jan's) which suggests that, as is often the case, that they were named for eg. Jan Hendrik but could have been called Hendrik all their lives. I have many instances of that. Sometimes, in large families (and 14 kids does qualify), cousins also share the same names which adds more fuel to the nickname fire... I suggest you look anywhere with a Jan or Hendrik as part of the first name sequence. Michiel Hendrik or Cornelis Johannes (Jan) in this case would qualify for example.
Another point to remember is when a second "Jan" pops up, often the first one died young(ish) ... or the name comes from the mother's and father's side. Also, when you look for a Death Notice, if a child predeceased a parent, the child may or may not be on the DN - info depended on the family. There may also have been a second marriage not recorded in SAG. (I found my one Kapp descendant who remarried and had another 11 children through a DN).
Here's what I can give you on the 14 KAPP children of Michiel Hendrik Kapp and Johanna Petronella Stander from SAG to help clarify:
Cornelis Johannes, b. 26.2.1861, d. 17.8.1910, x Mary Elizabeth RUSSELL, with issue (6); Maria Catharina Elizabeth, b.16.5.1862; Frederik Petrus, b. 1.9.1863, d. 26.8.1926 x Cornelia Elizabeth STROEBEL, with issue (6); Michiel Hendrik, b. 5.10.1864 x Anna Geertruyda MCCARTHY; Jan Abraham, b. 29.11.1865 x Jacoba Isabella BARNARD; Johan Coenraad, b. 28.3.1867, d. 6.6.1914 x Martha Catharina STEEL, with issue (7); Ernst Philippus, b. 17.7.1870; Adriaan Christiaan, b. 28.11.1871; Hendrik Jacobus Casparus Lamprecht, b. 21.2.1873; Johanna Hendrika Petronella, b. 12.6.1874; Susanna Elizabeth, b. 21.5.1877; Martha Jacoba Rosina Salmina, b. 28.7.1878; Geertruyda Elizabeth, b. 11.1.1880; Jan Christiaan, b. 1883, d. 29.7.1944 x Martha Dorothea VLOK, with issue (11) - one of which is a Hendrik Johannes.
Lastly, as valuable a resource as SAG is, it is still only a secondary source and does have errors. regards the Marais family - it is a large family and my lot from the E.Cape (Molteno) too so I can possibly help you there as well. I have a large database.
If there is more info you want, I suggest you try and get in touch with me. Unfortunately I do not have a family web site. Look me up under Bird Dog GD in the JHB directory. Line currently not working, so give it a few days.