AdaptedMind has a site for kids to practice their 1st to 6th grade reading and math skills. A one month free trial is offered.
Disclaimer: Be aware, that after the 1 month free trial, it is $9.90/month. Parents, if your kids love these games, then it can be a good value. However, be careful of continually paying if your kids stop playing.
For math games that are free without a trial period, see the other math games that I have listed. For an extremely fun, free reading game try teachyourmonstertoread.com.