Verbally encourage your kids whenever they listen/recite the Qur’an. Smile, let them know how good you think they are doing, and how proud you are of them. Try not to get them used to tangible prizes. If you do get them prizes, make it rare and spontaneous. You wouldn’t want your children to make a connection between reading the Qur’an and instant and tangible rewards. Rather, make it a habit to explain to them the virtues of the Qur’an such as healing and good deeds.