I also second this recommendation for the ASK Publishing readers, although they can be quite expensive to buy new. If you’re in contact with other Japanese learners in your community (or here on Wanikani) it might be worth seeing if anyone has a set they’ve outgrown that they’d be willing to part with. @Stillman777, if you don’t mind me asking, have you seen this particular brand of readers for sale in digital? I’d love to know more.
PIBO’s not bad for this either - you’ll need to manually look out for older kids’ books though unless you’re a subscriber I believe.