Supergirl's youth and inexperience make for a lot of interesting adventures with her getting into troubles that Wonder Woman would know better than to get into.
So you want an annoying kid sidekick? I'm not against sidekicks, but an inexperienced sidekick getting into trouble in a movie about the grown-up heroes usually doesn't lead to a mini episode of Young Justice, it leads to something like this:
- "Shiny green rocks, they're so pretty ...argh! Help meee!"
- " 'Magical death trap' Hah, like that can harm me, Super... argh! Help meee!"
- "It's so dark in here and it's just the two of us, Martian Manhunter, let me light some torches ... hey, why are you screaming ... NO, don't run!"
- "Tim and Conner are totally gay for each other, but they won't admit it, I'll just borrow Diana's lasso for a minute and get them in a room together, teheheh. *5 minutes later* Okay, Tim wants to do Conner and can't stop himself undressing him with his eyes every time they're together and Conner is ... not interested at all, awwwkwaaaard"