Infants when other Sims are not at home...

Realistically, daycare would be optimal, but you could work out that the kind neighbor is watching out for the kid with a nanny. Personally, I just use butlers, but they aren't exactly optimal unless your kids are from a rich family, or even a middle class family since butlers aren't all that expensive. I haven't had any problems with any of them, but I do use Better Nanny and Better Butler mods - the ones from LittleMissSam, of course.

Depends on the story you want to tell, honestly.