You Must Be Kidding!
I recently had occasion to be at a casting session for a DVD project. Three of the major roles could be cast using children or adults. The Producer wanted to try kids first as the characters are three small kittens. I wasn't consulted but I thought it was a good idea as I think it's always better if you can use kids to fill kids (or in this case kittens) roles. The problem of course comes if the role is too difficult for kids to give you a performance that rings true. There are a lot of times that what you want a kid to sound like is just that, a kid. Other times you want/need the character to deliver a more complex message to your audience - you may need subtly or just a certain inflection that tells a whole lot without saying anything really at all. These are the times that you may wonder if you would have been better off with an adult voice over actor reading the lines. Never an easy choice and I think you must first decide who is your audience. I think if you are making a film or television show for kids than you have more latitude; most probably the lines are not so long and demanding and you're not expecting a lot of nuance. After all, kids are not going to sit through long speeches that evoke provocative thought or question their world view.
The upside of using kids is that they actually sound like kids, not a forty year old voice over actor/actress (is actor now gender neutral?) trying to sound like an eight year old boy. Nothing worse really. It only works for very young audiences or if you have extraordinary talent taking the place of the kids. Another consideration with kids comes if you're producing a series. If the series gets renewed and goes on for awhile you may have to recast, kids voices do change and they may do so unexpectedly. If you decide to go with kids spend a few bucks and get a experienced casting director. They will save you untold headaches and time. There are a lot of showbiz kids and that means there are a lot of showbiz parents so be prepared.
At the end of the day you'll have to figure it out based upon your project and its needs. Even the best kid actors will need strong voice direction so be prepared to give them good reads and be patient. If you have a casting call through reputable talent agencies they should have done a lot of the screening for you. That's what they're there for so be sure they know what kind of character the kids will be asked to read for and give them as much information about the character as you are able.
Unless you feel you know exactly what you want and are confident giving voice direction, you may want a voice director to run the recording session. There are folks out there that do nothing else and they know how to work with kids. If you have time and the budget, you may want to arrange a table read with the kids before the session itself. It will help you get a better performance and the time you safe at the session will help offset the extra cost.
At the end of the day we ended up with several kids we thought might work for each of the three parts. Before we finish our development stage we will call those kids back in and ask them to read a bit more challenging lines to get a better idea of their range. In the first audition we had over thirty kids so it was not possible to give them all a lot of lines to read. Taking it a step at a time we hope to find the right three kids, but if all else fails we'll turn to old faithful, the handful of adult actors that we know can give us a great performance, even if they are forty-five years old....