I use Nextcloud with Carnet. I haven’t used Keep in years so I don’t know the current features but when I made the switch to Carnet it was very comparable.
On Nextcloud it is an app you install separately. For your phone you’ll find Carner on f-droid. Sadly there is no client for iOS. Link https://www.getcarnet.app/
Try https://code.org. It’s free and really well done with a low barrier of entry. There are different types of tasks for various ages.