OH. When you said steal, I thought you weer using the term in its more common (big league) sense. If you are following little league rules about ball crossing the plate, then I am doubly surprised you have this problem. Most coaches will tell the catcher to get the ball back to the pitcher ASAP because once he is back on the rubber the runner has to return to third. Just lean down and tell the catcher "you know son, if your pitcher has the ball, that runner's not going anywhere". He'll figure it out.
Parents - Can't live with em, can't shoot 'em. As long as she is not using profanity or saying/doing anything abusive or unsportsmanlike you're kind of stuck with her. Shame you can't toss someone for being annoying.
I have run into this once before in this age group at which time I just leaned down to the catcher, patted him on the shoulder and said, "don't worry son, you're doing fine". Kids with parents like that don't get adult approval very much and giving them a little tends to calm them down. The only other suggestion would be to suggest to the coach that they let the kid play center field where her voice might not carry.