Some more interesting points from Shmiras Shabbos K’hilchasa’s footnotes:
1. He asks the question: “If the point of lighting 2 is because of Shamor and zachor, why are you allowed to light more than 2? (he doesn’t answer the question here, but gives the source where it is answered.)
2. Some light 7 for the days of the week, and some light 10 for the 10 Dibros, and some light one for each child.
3. He says there is more than reason brought down for the minhag of lighting one for each child. One reason he gives is the reason that WTP gave earlier – a penalty for missing when you give birth. This is from the Mishna Halachos who proves from this that you do receive the penalty even when you are “ones” (“forced”).
So it seems that there are opinions that you get the penalty even if it wasn’t “shogeg” (by mistake). I wonder if anyone goes by this l’maaseh. It’s interesting that the Shemiras Shabbos k’hilchasa only mentions this opinion in passing, in a footnote.