@martinschlegel There are a ton of options! It depends on what language you're learning, but Mango is great and often available for free through public libraries.
Glossika is great for reading and listening, though less so for teaching grammar and is much more repetitive, so I wouldn't recommend it for kids but it's useful for adults. (It's a little expensive, but worth it in my opinion!)
Anki is open source and free on desktop and web and can be used for more than language learning (it's basically a fancy flashcard program), but I've found it's a really effective tool for practicing vocabulary. You can download decks other people have created to practice with or create your own, and you can even include images, video, and audio. The app is $25 which is pricey, but it's a one-time purchase and helps fund development of all versions of Anki. imo it could definitely be kid-friendly, but depends on the kid!