I had side arm for a while, but I corrected it by throwing a football...I don't know how it would work with your daughter, but it helped me alot. I guess becasue footballs can't really be throw unless your arm is up right. Hope it helps. (=

do the throwing drill slowly at first until the motion is second nature.
Start with the arms extended stright out while standing sideways, bring the arm up in the L shape and throw over the sholder while bring the glove hand back. Concentrate on the over the sholder action. Repete over and over until it is the only way she throws.
Like Kelly said a football is one way to do it.

Throw by a Fence it works because my teammate throws side arm and it works for her. Hope it works :) :)

This drill could help your daughter.

Softball Drills - Passed Balls and Wild Pitches

Purpose: To let softball catchers practice getting to the ball quickly and accurately and throwing it back to the plate.

Procedure: The catcher starts in a ready position at home plate in full gear. A coach is in front of the catcher and throws a ball behind the catcher, simulating a wild pitch. The coach can take the throw from the catcher or use a pitcher. The catcher throws off her mask in the direction opposite the ball (to avoid tripping over it) and runs back to the backstop, surrounding the ball so that her throwing arm is closest to the backstop. As she nears the ball, she should fall to her knees and slide toward it. Using the back of her glove as a brace, the catcher picks the ball up with the bare hand and sidearm throws the ball back to the plate. The ball should be thrown about 1-2 feet up the line at knee height. Each catcher makes 10 throws.

Coach Marc,
www.softballperformance.com

repitition, repitition, repitition. It took me about 3 months to get my daughter to change her motor muscle memory but is doing fine for the last 2 years and throws correctly.

i am a catcher and everytime i throw the ball i make sure my arm goes all the way over i do it for stretching but she could use it for that
i do arm circles and then i throw the ball like that and it might help maybe lol

Have her warm up in "serious" mode. I find most kids warm up without thinking much once they are 11 years old and older. But you drive home good mechanics only when you pay attention to then. So have her start from warm ups every day practicing good mechanics. Don't let her stop til you see 25 or more throws done correctly in a row. Have her do it almost like when she was just beginning - start all over with exaggerated correct movements - over and over. Habits are hard to break.

Kelly is right - throwing a football i a great way to force the mechanics, as is using the fence or a wall to throw next to.

I am a catcher too and i used to throw side arm too as well, until my coach one year came up to me and told me to go stand next to the wall (my throwing arm closest to the wall) and he went the distance form home to second and had me thow to him. Everytime I tried, i would keep hitting my hand on the wall, which hurt. This taught me to get my arm to go in the up and over, in the righ motion, instead of side-arming it. You should try, it really did help me.

i would draw a line on the ball and keep telling her pull back and let go by her ear

put her up aginst a wall and make her throw by her arm sliding on the wall