One of my favorite websites to use with young learners online is Arcademics.com. This is a free website that features interactive minigames centered on academic topics, thus the name Arcade + Academics = "Arcademics".
It allows you to filter by grade level from 1-6, or by subject and choose from a variety of topics. Students can play solo against computers, play with other students around the world, or log into private games with passwords they create to play with friends and family in real time! This website is totally safe for kids even though it allows interaction with people globally, as there are no chat rooms, and usernames are censored for age appropriate content.
As a tutor, I love setting up private chat rooms with my students and use the passwords as a chance to practice spelling difficult words. They can't enter the game unless they spell the password correctly!
This website has been a wonderful tool for me and I will definitely continue to use it for my elementary aged, and special education students!