You can find the app on iTunes for $3.49.
It's easy to use, just select the notes you'd like to quiz your child on and press "race". In settings you can adjust the amount of time that they have to answer, by choosing the speed of the horse (trot, canter, gallop). I find that for the little ones it is better to work with fewer choices, just a couple of notes at a time to start. You can always give hints to help them make the right choice. Some kids do not enjoy timed activities, so be sensitive to this.
Remember parents, it is your job to be positive!