She's bitterly disappointed if they don't.

As a parent, do I feed into this fantasy? It just seems so dishonest. Heck, I was never comfortable with Santa or the Easter Bunny.

awww, she'd probably enjoy The Borrowers books.

I'd tell her they aren't real. If she wants to believe, she'll just think you're a silly grown-up. I think. But then let her read books about things just like that.

I loved the Borrowers books, and my mom told me things like faeries and Littles didn't exist all the time. One of my favourite stories was The Brownies that would could into peoples' homes and clean them.

She'd tell me there was no such thing, then she'd suggest I "be a Brownie" and clean the house.

I was thinking of "The Borrowers" too. I guess I'd say gently, "It's nice we can pretend this" and then maybe even write an answer but I wouldn't make a big deal of the unreality of it.

If she wants to keep believing, and you don't mind, the Littles weren't keen on letting Bigs know they existed, why would they answer said letters?

The Littles was actually one of my favorite book series growing up.

It's hard to know what to advise here - I have met things that look like fairies and pixies so I'd be loath to tell anyone they're absent.

I'm also keen on letting little ones believe in magic, at least for a little while. Most figure things out otherwise as they get older on their own anyway.

Definitely let her read The Borrowers! You can say "These characters remind me of The Littles."

lego spacemen

Brilliant adults begin as imaginative/inquisitive children. So many ways to go here.

You could explain that all things do not communicate in the same way. Like dogs don't speak the same language as cats.

Encourage her to think of how best to help the littles hear her. If they don't respond she'll need to try something new until they do.

My guess would be that she should close her eyes and speak with her mind.

for a half formed idea it wasn't bad:
go ahead and write replies on her notes and put a lego figure with the reply