There were some very young kids at Sonisphere the other year (and they were seriously enjoying Metallica!). I wouldn't take them among standing crowds, personally, even if a venue/festival allows it. Some people did take young ones near the front, and although some people found it cute and funny, others got pissed off that they had to watch where they were standing all the time and couldn't jump about. You'd best work out if they can stand very loud noise, too. Maiden indoors are pretty damn loud.