Comment by debunn

Might I suggest https://www.prodigygame.com - it's a free-to-play online math game, where your child is a wizard that has to answer (age appropriate) math questions to gain magic to cast spells. Note: there is a paid subscription that allows your child to get access to more pets / faster experience gain, but is not required.

While this isn't a "do math to be able to unlock your device" type of game, it is fun to play and can be used as an earned screen-time requirement (or a "free screen-time" option!)

Disclaimer: I work for Prodigy as a Site Reliability Engineer, but my son (10) also enjoys playing the game!