Depends on the kid(s), age, progress so far etc i would think.
Up to high school algebra level I can talk a little about but ymmv. Everyone is different.
The number Devil is an enjoyable story. Khan academy has a good problem bank with gamification of progress etc. Mathantics is a good substitute for school teaching of conventional stuff on video and has good worksheets. Mathific app is another source of practise and gamified progress.
Interested to hear other ideas and for other ages.
When my daughter is a bit older I’m going to buy her the Singapore school books for young math and pay her a salary to complete them. I don’t doubt their effectiveness.
