Everyone could guess that the upcoming Battle of Seven Potters was going to be dangerous. After all, they were probably going to face Death Eaters. Why were a bunch of school kids chosen for this? Why didn't skilled wizards (like Aurors) use Polyjuice Potion to become Harry?
They weren’t children, they were all adults, and they all volunteered.
As Moody says to Harry, he’s the only underage wizard in the party (ah, the perils of a July birthday); everybody else is an adult:
“If it has to come to force, then it will,” growled Moody, his magical eye now quivering a little in its socket as he glared at Harry. “Everyone here’s overage, Potter, and they’re all prepared to take the risk.”
There’s a lot of experience among this group.
The party includes:
- four members of Dumbledore’s Army (Ron, Hermione, Fred and George)
- three Aurors (Kingsley, Moody and Tonks)
- a DADA teacher (Lupin)
- a Triwizard Champion (Fleur)
- Arthur & Bill Weasley, Hagrid and Mundungus, all of whom have considerable experience under their belts
A party comprised of first-years might be more of a concern, but this seems like a pretty solid group to me.
They need to keep this plan a secret.
Everybody in that group is pretty trustworthy, and most (if not all) are members of the Order (I’m not sure about Ron or Hermione). We know they’ve leaked a fake trail to try to shake Voldemort off their scent:
“The one thing we’ve got on our side is that You-Know-Who doesn’t know we’re moving you tonight. We’ve leaked a fake trail to the Ministry: They think you’re not leaving until the thirtieth.”
For this to work, calling in a pack of Aurors is out of the question because that blows the false date immediately. Since they had a group of competent volunteers from the Order, all of whom could be trusted absolutely, calling in external help would be an unnecessary risk.
As for why it was the younger members of the party who took the Polyjuice Potion, consider that transforming into somebody else is probably quite disorienting. The body you’re in reacts differently to your usual body, which means your instincts (in particular, your fighting instincts) are probably off somewhat. If you’re Harry, you have the added complication of poor eyesight:
“Harry, your eyesight really is awful,” said Hermione, as she put on glasses.
Even with glasses, it probably feels different to looking through your normal eyes.
The adults are the more skilled defenders of the group. You want them fighting as themselves, because you want them at full readiness, not handicapped by Harry’s different eyesight and physique. The defenders are more likely to be engaging with the Death Eaters one-on-one, not the Harrys. Ergo, you put your adult fighters on protection duty, and the young ’uns as Harry clones.
Additionally to alexwlchan's answer, I want to add this:
“I’m a soldier, I’d sooner be a protector,” said Mundungus. “Shut it,” growled Moody. “As I’ve already told you, you spineless worm, any Death Eaters we run into will be aiming to capture Potter, not kill him. Dumbledore always said You-Know-Who would want to finish Potter in person. It’ll be the protectors who have got the most to worry about, the Death Eaters’ll want to kill them.”
Therefore, even if the younger ones (Ron, Hermione, Fred, George, Fleur) were chosen because of their experience and trustworthiness, they would still run less risk than the more experienced ones (Kingsley, Moody, Tonks, Lupin, Arthur, Bill, Hagrid).
Aurors did participate, Tonks and Moody among them.
I believe all participants were of legal age (at least 17), which included Harry, Hermione, and Ron as well. Fleur, Fred, and George had also 'graduated' from their respective schools and were viewed as adults in the wizarding world, therefore able to give their consent and risk their lives. Same as being able to enroll in the armed forces at age 18. To top it off, as the Golden Trio had already decided to forgo their seventh year at Hogwarts, none of the participants were 'school kids', either.
well also remember, Hermione is almost 18 when Book 7 starts. Since she was almost 12 when she started Hogwarts-she's born a year before Ron and Harry in the middle of Sept. Ron's birthday was in March-the day he act those chocolate Cauldrons Harry got and ended up having a a bezoar shoved down his throat?