IXL

 IXL is a comprehensive online tool that provides math, language arts, science and social studies practice. The content ranges from PreK-12. PreK-1st grade only has the option of language arts, and math. For second grade and up, there are also science and social studies exercises. Kids can try the skill some skills for free, but to get the full access to the website you must pay a membership fee. Membership fees range from $9.95-$19.95 per month and $79-$159 per year based on the number of subjects your child wants to learn. One really cool feature of the website is the Continuous Diagnostics  tool. This is a great feature for parents, students and teachers. This page shows what the student has already completed, and uses the data to show what exercises the student should complete next. Another good feature is that after the question is answered, the student gets immediate feedback. If the answer is wrong, then there is an explanation as to why the child got it wrong. I work in a kindergarten classroom, and we use this website. The only downfall for younger children is them not knowing how to properly use a computer/laptop. The site does give verbal directions for each question, it's just the matter of whether or not the younger ones know how to click the button so it can tell them what to do. I would recommend working in small groups if you are working with young children on IXL, so you can guide them through if they need help. IXL has an app  for iPad, Android, Kindle Fire, and Chromebook.