A few things here. It is very possible the either/both of Naruto's children will inherit the Byakugan (learn to genetics), on top of which it is more than possible that they will also inherit the Sage's body (in the same way the Senju and Uzamaki did) making them powerhouses in the ninja world... not only that but it is also feasible that the chakra of the tailed beasts could be given to the kids as well, do you really think Kurama would allow what are essentially his children (due to being a part of naruto for so long) as well be weaklings? Not to mention the tailed beast chakra in Naruto hasnt be "sealed" inside him for quite some time, it is free to come and go as it pleases, so why wouldnt the TBs willingly give/allow some of their chakra to be passed down to their first and only friend's children? They love naruto and they want to see his lineage stay in charge so that they have the chance to live in peace. Also, nobody knows what the effects of having children while having all the TBs inside of you does (the last time it happened, Hago had Asura and Indra, so im betting it has a profound effect on the children)