Pressure, Cover, Balance, Balance, Balance - This sounds familiar

We still struggle with balance. My U10 girls, from time to time, just get mesmerized by the ball and forget to mark the girl in front of our goal. It's not all the time. A lot of the time they get it right, but a lot of the time they just don't look around to see if there is a player that we should be marking. Maybe I'm expecting a maturity level I shouldn't expect. Maybe I should draw a line down the middle of the field and tell players not to cross it. I want to give the girls maximum freedom on the field. I want it to be fun more than anything else. I want them to do the right thing because they're taking care of each other rather than following a rule. However, I want to stop these easy goals too! I'm open to suggestions.