HELP! What do you write and how do you know it's a good fit?

So I've been writing this middle grade book... (it's called Parasite's Don't Wear Pink--about an only child adjusting to the idea of having a sibling)  and I love it.  The word count over there is creeping up and I'm almost done with my first draft!  Huzzah!    

I like writing from this eighth grade perspective, because it was a big year for me--I think it was the first time I tried to figure out who I was/wanted to be...(this happens every few years or so).

I'm enjoying it so much, but I don't know if I'm getting it right.   So how do you know?

Here's what I've done so far...

1) I have read a million middle grade contemporary books. I love upper middle grade, it's just fun.

2).  Researched writing MG through blogs, etc.

3.) I'm trying to find a few MG crit partners... (If you need a CRIT PARTNER and you write or read MG, let me know, because I'm still searching...)

When I wrote Etched in Glass...I thought I'd always do YA, but I've had so many ideas lately that just seem like they'd work better for an MG audience... the problem is...I've heard so much about nailing an MG voice and how hard it is... so how do you know??

Why did you pick what you write?

Do you switch it up?

How can you tell it's working?

Seriously I need to know.