Computer games/programs for 3rd-6th grade

In our schedule I have each of my 3 kids rotating through 30 minutes of computer time to give me some 1 on 1 time with the others. I have found Reading Eggs for my 5 year old and love it. My girls have been using the free trial of Reading Eggspress and I don’t love it. They just goof around and don’t seem to be actually learning much. AND I would have to pay for a subscription for all 3 kids! I thought one would cover them all. I am getting it for my son because he learns quite a bit. What are some GOOD games for older kids? Low cost/free is even better!