Just trying the hemmingway app. It's a neat little device that catches readability errors. My only qualm with it is that it assumes if a word ends in 'ly' it's an adverb (it misses a few like only, but apparently 'apparently' is an adverb?).
Apparently I write somewhere between (US) grade 4 and grade 6 level. So a 10-11 year old should be able to understand my books.
Unfortunately, they don't have a Linux version.