I suggest you to make them regularly practice a little Pranayama and to meditate daily. They will not feel the need for tuition classes anymore. If you really have to, then one hour of tuitions is more than sufficient for them. To enhance their intelligence and skills, you also need to pay close attention to the kind of food they eat and their behaviour. Do not give children fried foods to eat right in the morning. Make them have lots of vegetables and fruits in their diet.
Adopt Ayurvedic techniques and give them herbal medicines like the Medhya Rasayana (a nervine tonic for boosting memory and reducing mental stress); give them milk with Ojasvita (a milk powder made with combination of seven powerful and nutritious herbs). So encourage children to take such good food and supplements to enhance their intelligence and skills.