My sons have been using Duolingo to learn French and German for about a year now, but a few days ago my oldest told me there was a new “dashboard” for teachers to use as a classroom. I looked it up and it’s also amazingly free! My sons have been using the totally free site for the past year and have found it to be very good at motivating them to continue learning every day. It plays a bit like a game, so there are points to get for completing lessons and translating articles. They showed it to me when they found it to learn German and have since started learning Norwegian, and Spanish as well. They tried Russian but found it too much to learn along with other languages.
You might try just putting your kids onto it and seeing where they go with it, but if you’d like to be able to assign work with a due date, you can log into Duolingo for Schools
and sign up your school and invite your kids as students. It’s very easy to use and completely free. Pick a language to learn together and speak to each other at the grocery store like it’s some kind of secret code!