Kishimoto explained it during an interview - he simply forgot to give it to them. This is so stupid of him if true...
This is the relevant section:
キャスト陣からヒナタの2人の子に関して「白眼」は? と質問が飛ぶと「やろうと思ってたけど、忘れてまし た(笑)。(子どもが)3人になれば…。でも、そうなるといろいろ複雑になりそうで…」と苦笑 していた。
(On mobile, so will have to give loose summary translation~)
Interviewer asks, "Why don't the two children have the byakugan?"
Kishimoto says, " Ah, I intended to, but I guess I forgot about it (laughs). If there's a third child, however, but then it seemingly will get complicated in various ways (laughs sarcastically).
So uh, no. Not really going to have another child. It's more like if he wants to fix the mild error and give the byakugan to one of their children, then he's going to have to ask NaruHina make another one lol~ (The hypothetical third child would then have it).