I always thought the book I am writing would be aimed at a mature children's audience, but then sometimes I look at it and I think it's more 'young adult' as it could appeal to an older audience than the original 10-15 age range I imagined. When I compare to, say, Harry Potter, I still think that JK Rowling's work is mature for her age range, so how do you decide? When it comes to audience, what is the best way of labelling, sometimes, a book that could be 'across the range'?
I'm actually a teenager myself and I have to say, that while having a set target audience or age range you image would read your book is good, it isn't going to stop people who are older or younger from reading the story you've created. Some parts of the story may seem more suitable for children while others may appear to be directed at older teenager audience, and that may actually make the story seem better. You used the example of Harry Potter, in J.K Rowling's story, she aged the books with the group of people who would have been reading them when the first book was actually published. Making it enjoyable and more understandable for the people reading the story and allowing the newer audience to feel more mature while also being able to read things that were still suitable for their age range.
I think it clearly is fantastic ingenuity and humor to captivate a child reader.
I say: you don't. You pick your audience and anything else is a bonus.
When you look at JK Rowling, it's true her books have a lot of crossover potential, but looking at the first book in isolation it's clearly aimed at children ages 9 -12. The second book as well. It's only as the series continues and the characters age that the audience changes as well.
10-15 sounds like an odd age range to pick (I'm not sure if it even *is* a publishing age range). Consider that a 15 year old may be sitting their GCSEs wheras a 10 year old isn't even at secondary school yet. Would your average 15 y/o want to read something which is aimed at a 10 y/o? I was acutely sensitive to childish things when I was that age and desperate to be seen as an adult.
According to Wikipedia, YA is 12 - 18 but when I was that age the teenage section was labled as 14 - 19 which sounds more likely to me - I wouldn't hand The Hunger Games to a 12 year old.
My advice is: pick your audience and write for them. Trying to make it appeal to both is a sure fire route to being too mature for one and too babyish for the other. It all comes down to the subject matter and how it's dealt with.