Children, once they're grown

[milo]

Children, once they're grown

Children, once they're grown,
are gone forever.
Where do they go to?
Why does time never stop?

So, I have a couple issues with this.  First, The title adds nothing at all, so it is a wasted opportunity.  But second, and most importantly, the whole first strophe is just exposition and it isn't new exposition, readers have heard this sentiment 1000 times and most of them are better said.  I really feel like you need a metaphor here to draw the reader in.

Why can't it be like how they
told us of the future
in comic book shops - 
where the past is a page we could visit once again,
not in the mind, as through a window pane,
but walk in once again?

I don't think bring in the royal "they" is helping at this point. I think here we finally meet your metaphor so, not surprisingly, it is the best part of the poem.  The past like pages to flip through and memories like window panes.  Unfortunately, it is drawn a little too clunky.

In Browning's retold rhyme
the piper wasn't death, only the 
passage of time.

Yes, Browning did already say this, it was a masterpiece of writing.  In this case, I don't think the allusion helps that much because it feels like it overwhelms your whole story

Tell me there's a way 
to come back from tomorrow.
Time is at the quantum level
a double headed arrow,
merely an illusion, pure poppycock.

The immediate dismissal of the narrator's discourse doesn't allow the reader to fully absorb it before it is immediatey irrelevant anyway.

So can you turn back the clock 
to just my boy and me?

yah, "turn back the clock" is just cliche so why add it here?  you have already said as much several times in this poem?
The ending feels like it could have been a reveal but lacks punch

Thank you for posting this poem