A reward is simply a goal that has been achieved. There are two ways to redeem your rewards. The first is when it is 'achieved', that is when a child reaches the number of thumbs allocated to reach that goal. When a goal is achieved, a screen is shown with the number of thumbs allocated to the goal, the time it took to achieve the goal, and if repeated, how many times it has been repeated. If you hit 'close' on this screen, the achieved goal will be stored as an Available Reward, if you hit 'reward now' > ok, it will be stored as a Redeemed Reward.
To find the stored rewards, go to the Dashboard Screen (this is the second icon down the bottom of your screen that looks like two purple squares on top of one another with a purple rectangle next to them), and tap on the Goals Tile (top left). Immediately under your child's name you will see the Goals and Rewards buttons. Tap on the Rewards button. Any stored Rewards waiting to be redeemed will be shown here. Tap on a goal to redeem it.