Certain 90's Animated Kid's Movie?

Question

There was this movie from when I was really, really little that I loved, but I can't remember what the name is.

It was animated and was about little woodland creatures. I think they were mice or rabbits or something. Anyway, I think they were at school when these people came in and sprayed something, gas I think, in the forest and the main guy's little sister got really sick from it. He ends up having to go and find the flower that would cure her. They make a flying machine and I think there might have been an owl in there someplace?

If anyone knows anything else, please help! This is driving my friends and me crazy.

Answer 1

Oooh I forgot all about that!  I saw that too I think!  Sorry X_X hope someone else knew/knows the correct answer.

yay! I'm ganna rent it on netflix that was a great movie.

Answer 2

I think the title starts out Once Upon a Forest

http://www.imdb.com/title/tt0107745/

Answer 3

I almost said "Ferngully The Last Rainforest," but on second glance, that doesn't seem to fit.