This is a plot/worldbuilding question: How much time do you think passed between Asriel and Chara's childhood, and the events of the game? 

I'm asking because I think it'd be interesting if we could interact with child versions of the main NPCs, but that might not make sense timeline-wise. And we could stick with Toriel and Asgore, or make OCs.

Plot theories below. Sorry it's so long, I got a little carried away, so you can skip to the tl;dr if you want.

In New Home, there's a calendar. In Neutral/Pacifist, its description is (There's an old calendar from the end of 201X.) (A date is circled on it.) but in Genocide, it is (The date I came here.) meaning when Chara got there. The beginning animation is also marked 201X, which would indicate that Frisk fell within a decade of Chara. However, it seems unlikely that Asgore could collect 6 human SOULs in less than 10 years, or the monsters would've collected enough to break the barrier way earlier. Also, the main NPCs didn't seem to know Toriel, which is odd if she left just a few years ago. The alternate explanation is that the beginning animation was of Chara falling, and that Frisk came along many years later. This would explain Alphys' extradimensional phone - she didn't invent a cellphone that could access dimensional boxes, space-warping phones were a common technology by that point in the future. However, if it was far enough in the future for the 6 SOULs to make sense, Frisk's time would be way later than ours. The world we see in the True Pacifist credits doesn't look very future-y. And Gerson (the turtle in the Waterfall shop) can tell you a story about Toriel and Asgore. Thoughts?

tl;dr Either Frisk and Chara fell within 10 years of each other, or Frisk is from the far future and Chara from 201X. Or something I didn't think of.