As the 10 year old boy that I was when I watched Johnny Quest, girls don't go on adventures

.
No seriously, the show in the 60s was supposed to be cool for little boys, and was purely that. They tried to wedge in a girl in the 80s to get a bigger audience and thus more money. They need to stop compromising things to make everything appeal to everyone.
I kinda look at it two ways.
1) You'd want a girl cast to avoid a "sausage fest".
2) To make the story more interesting, and the danger more "dangerous", you'd want to cast young, pre-teen kids, like in Jurassic Park.
...But when you consider the latter, it doesn't really matter if you're avoiding a sausage fest, huh.
But basically, if you don't want girls on adventures, you don't want kids on adventures either. That's thinking as an adult. Unless you meant, 10year old boys wouldn't want girls tagging along on the adventure, that kinda makes sense. But only slightly.
I wouldn't have minded having a girl along on an adventure...
The other thing i think of is, the Quest team never really seemed to go looking for trouble. It mostly found them. So, if they were worried about a girl bothering them or gettin in danger on an adventure, it was never their fault.
In the 60's it would've been a compromise to add a girl to a boy's TV show. Today, it'd just be common sense. I know of one (male) buddy who purchased 300 on DVD, while I know of three girls (and they can be girly girls at times) who bought the movie and flipped out just as much as the guys at the action scenes. While "300" was definatley a mighty, "Guy's" movie, with two (womanized) female characters (and whatever Xerxes was..), i think girls/ladies will come to see this movie too.
It would just go over better if a girl were involved. As long as she weren't "Kim Bauer-ized", I'd be cool with it.