So it goes to follow it probably doesn't matter too much if you read one out of order. Not that I would choose to but if it just happened that way, it wouldn't trouble me. That's adult think! It troubles kids a lot I discovered when I read Jayne Lyons wonderful 100% Wolf series to #2 son out of order. (Brief promo: this is werewolves and creatures of the night without the scary stuff. Perfect for younger readers who want the action and adventure of horror without the fear factor. Our hero sometimes has problems during his transwofation and turns into a pink poodle. Always at the most inappropriate moments).
I read the second book first - 100% Hero. It came to me via a friend who received it for review. (Great recommendation Barbara). It was great fun and #2 and I thoroughly enjoyed it. When I attempted to read Book 1 I was interrupted with - "but we already know this". I had overlooked a critical fact. While the story hadn't been told, it was already given away. I had to argue really hard- my best lawyer's defence ever - to be allowed to read 100% Wolf. I had to promise never to read books out of order again.
I'm often asked if it matters if a child reads a particular Samurai Kids book first. In the past I've said no. After all, each story is complete in itself. But from now on I'm going to answer differently. Start at book one otherwise they might never go back to the beginning.