Cute, though their daughter(naruhina's) having those markings on her face seems off. As far as I can tell those marks are a result of exposure to Kurama's chakra in Kushina's womb, as the only other people in the series to canonically have the marks are Ginkaku and Kinkaku of the cloud. Both os which were swallowed by kurama and started eating him from the inside. So I doubt Naruto would pass on those marking to his children, save for maybe hinata receiving a high exposure during pregnancy.
Yes, that's a Sai son. Well, I would agree, you'd think the NH child would be with a SS child, but that's such a common choice in possibility. Just because their parents are friends or are close, can we be entirely certain that ensures their children getting married, together? Things could happen, or there could be another chick or guy the other pursues, etc.
Somehow, I don't think Naruto is a frail old man, I can't see the fox letting his body deteriorate, and he'd find a way to share that with Hinata so they could function well and long to play with their granddaughter.