Did Nobita marry Sizuka?

But as the story goes, Doraemon was sent by Nobita's grandchild when he came to know that his grandfather was a useless, under-confident, and lazy chap in his childhood.

Doesn't that mean that the grandchild was the grandchild of Nobita and Sizuka?

I think Shizuka married Dekisugi. And that's what Nobita's grandchild wanted to change. That Nobita becomes smart enough to marry Shizuka. That's why he sent Doraemon.