Several things can make it difficult for a bike to maintain speed. Unfortunately children's bikes are not always efficient, but there may be things you can do.

Is she riding strictly on the road? If so, switching to slick tires instead of the knobbies that a mountain bike comes with would be very helpful.

Ride beside her and watch/listen for anything that may be rubbing. Anything making a sound other than the cassette's normal clicking could be a problem.

Also, if there is suspension on the bike, and it is adjustable, tighten the adjustments to make it as stiff as possible, suspension softens the ride but it sucks away power.