We played Set with my son when he was ~6.5yo, and he immediately loved it. After a couple of games, he was spotting sets almost as quickly as we were. But his performance and interest varied based on distraction, tiredness etc.
If you want a one-player Set game, there's a nice open source one here:
The 'AI' looks for solutions randomly. Every 2 seconds (in easy mode) it picks two of the visible cards at random, and sees if the card needed to make a set is visible.
If your kid is totally new to Set, you probably want to adjust the delay to 5 seconds or something.
Set is more fun against human players of course.