Hi. I would not post your work at a site like you describe. Not only would they be free to use it, but your work could then be considered published with first printing rights lost. Many publications only take first rights which would mean you wold not be able to submit this particular piece to any of those markets.
Regarding the word "irk" - that is not a preschool to 1st grade level word. I have a great book titled "Children's Writer's Word Book (2nd edition) put out by Writer's Digest Books which has word lists grouped by grades K - 6th, and "irk" is listed as 6th grade level.
As far as online word sites, there may be some, but I'm not familiar with any that I could recommend. If you plan to write for grades K through 6th grade I would recommend the book I mentioned. I believe I bought it at Amazon, though it may be available at Barnes & Noble or other places as well. Most of my children's writing has been for grades K - 6 and the book has been invaluable. When writing for children word choice is very important, and in poetry it is especially important.
I hope I've been of some help. Good luck.