Are your devices likely something that they would have fun with and choose to engage with or is it likely to be ignored unless adults use some kind of persuasion to have them use it? Is it cool with a child using it for a bit and then not using it for a few months and then wandering back to it? How far up into math does it go compared to what an a randomly sampled adult could actually do mathwise? Also for reading, are you using phonics or whole word sighting? For math, to what extent is it screen manipulatives versus manipulations of digits? Also, do you have provision for an older child to start learning this stuff so the basics need not be at a 4 year old presentation level, but the concepts still need to be covered?
In Sudbury schools, the typical age of self-taught reading is 7-9 though it can range from 4 to 12. Useful arithmetic usually seems to happen much earlier than reading though reading tends to get completed by the children on their own while arithmetic does not advance further than the needs of money exchange without special effort. In the long run, Sudbury students have no problem with college level material, including mathematics, but it could be nice to have something that eases the white knuckling if it does not undermine the child's self-directedness.