The Potts mod setup seems to reduce braking power by running the cable down through the stem bolt but there are a few things you can do to help:

Pull the cable out of the housing and grease it up or better yet, replace it with a high quality cable that's smooth wound and has a lined housing. While your at it, lube the brake and lever pivot points. Cleaning the rim with alcohol and installing better quality brake pads will help too. Running the cable directly from the lever to the caliper will help but you lose the capability to rotate the bars 360 degrees.

If we're talking about something from Toys R Us or a Wal-Bike, keep in mind that many of these come through with cheap flexy brakes with plastic levers and there isn't much more that can be done there to improve the stopping power.

Unless she's going to be pulling off flatland tricks, you don't want to have a brake on a kids bike that can lock the front wheel. That's just asking for trouble.