Lalaith

Lalaith she was called, Laughter in our tongue;

This line tripped me up because it's rhythm is so different from and it's much longer than the first two lines. I think it might work better if you broke it up into two at the comma; it will give the reader a visual cue that the rhythm has changed. I also think that switching "she was" to "was she" would hearken back more to Tolkien's style.

Upon the grass she twirled and sung.

And then we're back to the same rhythm as the first two lines. This constantly shifting is a bit jarring. One suggestion is to rephrase this line and the previous to something like this:

Upon the grass she'd twirled and sung --
Lalaith, named "Laughter" in our tongue.

For the Evil Breath cut short the laughter and the song.

Again, the rhythm and flow were thrown off by the sudden change in line length/rhyming pattern.

Overall, it's not a bad start, and it was nice to see a poem about one of Tolkien's lesser-known works. :D

Hello there, Laura!

Here as requested.
Suggestions in blue
Errors in red

Treading softly, treading lightly

Treading is a good pick for a word but not really for the first line of this piece. 'Walking' might be too cliche' also. Maybe choose words that will complement the poem but don't worry, treading is fine.

Upon the grass she twirled and sung.

Nice.

Lalaith she was called, Laughter in our tongue:

A very unique and nice name, by the way. What's with the colon anyway?

For the Evil Breath cut short the laughter and the song.

I'm not saying this is wrong but basically you don't need to capitalize those letters. I think you were doing it for an intentional emphasis.

And sorrow has begun which shall never be amended!

Good ending. An exclamatory line, I see.

Aside from those nitpicks, I must say I like it! It was poetic, not-a cliche' poem. Quite exceptional, really (but in a good way).

Keep up the good work!

Peace out!