I was recently given the opportunity to review the La La Logic Preschool Curriculum with my 4 year old daughter. You could start using the program as early as 3 years old, and the program is geared for children from ages 3-6. La La Logic is $29 for a lifetime subscription, and it has a free trial you can try out for one month as well.
La La Logic is based on IQ tests and encourages critical thinking skills and problem solving for your children. As my daughter is only 4 and I haven't done a lot of formal school work with her yet I was amazed at how well she did and how quickly she picked up new skills!