I don't believe there are any missing parts in the original examples. To my knowledge, it's the comparative that technically requires a complete comparison (which is oftentimes omitted). The superlative can stand alone.Hi, Ariel! My guess is, both these sentences have missing, implied, parts. The complete sentences would read:
He's nicest of all the nice persons he can be (or is) when he's with children.
He works hardest than all the instances in which he works hard when he's doing something for his family.
If the comparative form is used then the implication is that only two ways or degrees of being nice, and only two instances of him working hard, are being compared. So the sense (or, at least, the emphasis) will change slightly.
You shouldn't have deleted your post; the so-called omissions were quite important to make clear where the superlative meaning was.You are right, earthmerlin. The parts added by me (in red) were not omissions in the sense that their not being part of the original sentences made the sentences grammatically incorrect or vague in meaning. I only meant to show how the sentences could be expanded to reveal the reason why superlatives had been used. But you are quite right. I would do well to delete the post since I now see others had already responded in far clearer terms to the posted question. Thanks!