Once upon a time, it was only fantasy writers who thought about series, or more usually, trilogies. It was thought harder to sell a stand-alone than a set of three. If you were a children's writer, you stood in awe of R.L. Stine and series like the Babysitters' Club and Saddle Club. How would it be to write 50 or 100 books in a series? Then in Australia, we got Deltora Quest by Emily Rodda - the perfect series that was really a serial. The kids had to buy all of them to find out what happened. I actually had a bookseller point to them one day and say, "Look at that - $14.95 each. You should be writing something like that."
I wish! My first book ever - The Too-Tight Tutu - was one of the first six in an innovative series published by Penguin Books. They called them Aussie Bites, and each book had a bite out of the corner of the book. The series had a reputation for great chapter book stories right from the start, and there are now more than 80 of them. They've been followed by Nibbles and Chomps, and they're all different from what was usual then because there are many different writers and different kinds of stories.
The joy for writers is that you're not locked in to one concept, and the series are also often open to unsolicited submissions (but you should definitely read 10 or 20 of them first). The other side of all this series stuff that I see now, however, is that everyone who wants to write for kids thinks their way in is via a series. I'm sure publishers do want to discover the next hot thing, and sell a squillion, but it seems like many writers aren't doing their homework on this.
Have you, for instance, gone and sat for a couple of hours in a large children's bookshop and analysed 12-15 different series in the age group you're aiming at? Have you bought at least half a dozen of the ones you like best and taken them home for a critical read? Have you looked at genres, and at what's hot now? Because if it's hot now, by the time you get yours up and running, that trend will be on the way out. Have you actually thought beyond the first book?
Years ago, I attended a great conference that focused on writing sitcoms, and I still remember many of the key principles for a series that they covered - which are the same for any kind of series. Can you come up with 20 ideas for plots? Is your first story one that successfully introduces a great cast of characters? Does your concept fit the lost dog story? Does your series concept fit into the target age/audience you're aiming at? Is it new and different? And if it's a success, can you write 100 of them?
The problem with stand-alone novels at the moment is that they are being swamped by the series - I've heard at least two publishers admit this. Marketing doesn't want to commit to one book that may well disappear as soon as it comes out. But on the other hand, marketing doesn't want to commit to several books in a series either - what if the first one flops? Oh dear. Are you starting to feel like the ham in the sandwich? I'm not sure what the answer is. Maybe it still comes back to just writing the best book you can, and keeping a tiny window open for at least one or two more. What do you think?