Children taken away from their families and off the street are experimented on to become weapons in a unseen war. Only one, a young girl, survives and becomes a force to be reckoned with.

You are missing an important element, the lead character's goal

Try re-writing this logline from the lead character's point of view

"When she is kidnapped by a secret organization, a local street-girl turned assassin must.." (Then add her goal)