Buy a squeegee and tell your niece to squeegee the shower and the shower door (if you have one) after every shower. That prevents the build up of soap scum.
You can also just use a towel to wipe it down after each shower. With this method you also have to have a place to hang the towel so it dries during the day.
I do this every day. In addition to preventing unwanted buildup, it stretches just about my entire body. Two for the price of one!