@Kegaten I read this a while back and had to read a few chapters and synopsis to get a refresher. This extra chapter is not considered canon by the author. There's a lot of ways this chapter can be unpacked. Since she is living by herself, she probably doesn't have any offspring or husband at this time. I'm assuming that Chihiro is dead at this point of time, and being a partial zombie, he should have lived for quite some time. So these children are many years down the line. Due to the boy's interest for zombies, he is probably the great-great grandchild of Chihiro while their appearance resembles Ranko. So that's just confusing. If the effects of time was quicker than expected, than the grandmother would have been Ranko and the grandfather would have been Chihiro (hence why she said to say hi to their grandmother for her). But there are holes in this thought process and considering this isn't canon, we can just enjoy this chapter and not think too hard about it.
Let's look back instead at the last chapter of the series, where Chihiro and Rea meet. Because Chihiro is becoming like that of a zombie and Rea is a zombie and they are obviously crazy about each other, we can assume they end up living together for the remainder of their lives, especially with the end line of the manga of something like "together until we rot away". If I link this to this chapter in question, I would think that the boy's great grandparents, one being Ranko and the other someone we don't know, knew of Rea and Chihiro living together and visited them often enough. Chihiro's love for zombies and Rea being a zombie would have rubbed some interest off on one or both of the people. Chihiro eventually passes away and Rea decides to live in seclusion, most likely due to grief, like she was doing before. So the future generations don't get to know about Chihiro and Rea with Rea becoming a myth/legend/folklore, but the love of zombies still gets passed down Ranko's descendants. At least that's how I see it.