IMHO, age isn't such a big deal for the care of the animal. I believe it is the parent's responsibility to double check that the animal is being properly cared for, whether its a dog or fish or mantis. Age is a critical factor in handling however.
For species, I think ghost mantises are a no-brainer. They get to be an appropriate size; large enough to be interesting but small enough not to be scary. They are active and eat well, but do not seem prone to getting startled and jumping/flying. I have never had one of ours strike at me or bite except when I was pushing it to climb for feeding. They grow and change rapidly and have great camouflage, both of which can be good lessons for kids.
I think inverts generally make great pets for kids. Their life-cycles are generally shorter to align with kids' short attention spans, they are cheap, they are relatively easy to care for, and they come in so many varieties. Isopods, snails, roaches, mantises, beetles, millipedes, hermit crabs, etc. are all good choices, and to me, are more interesting than the typical hamster or (usually poorly kept) goldfish.
