Chapter length has been addressed in a recent post - please skim the first page or so of threads to see if your question has already been answered. Their general consensus is that there is no strict length - as long as it contains a important plot point, then it's served its purpose.
Second question is partly answered by yourself: you should just KNOW. No one can be a better judge from the outset about what age range the book is targeting, or how close to the target you're staying. But here's something in case you really don't know:
TOO HIGH
Subject matter too mature - 9 years olds shouldn't be target audience for a Hunger Games-esque novel.
Language - both in terms of vocabulary AND swearing; unless they're heavily into steampunk or a Series of Unfortunate Events, kids aren't likely to want to wade through excessive prose. And unless you're VERY good at characterisation and VERY lucky, don't trying cursing in MG.
TOO LOW
Taking Down - Kinda the opposite of my last point: if you've decided on a target audience, then don't forget they're just that - your potential audience. Take care to use appropriate language, structure, dialogue, etc.
As for word length, anything goes if you're good enough. Guys like Morris Glietzman can get away with shorter books because they always turn out diamonds. But the general accepted length for MG novels its anywhere between 33-45k, last I heard.
Hope this all helped!
Dave
