Is there a gadget or online tool that will assess the reading level of a sample

[RebeccaGirl2007]

of writing? I am writing a read-aloud picture book for kids. I want to know the reading level of the text I have written- not just vocab, but also the syntax and sentence structure. Twenty years ago I had a word processor that not only provided a word count, but would also instantly assess the reading level. I use Open Office right now- can't afford Word-- is there a gadget or a website that can help me out?