I didn't write about the Jeremy Renner app because how can you improve on perfection? After the dust settled, though, one of the good folks at Uproxx wrote a very good fancy think piece about it.
Byrne's Euclid is a strange Victorian edition of Euclid using colours instead of letters. I'm not sure whether it's really easier for children, but it is interesting to look at. Some excellent person made a classy online edition.