Comment by hnbad

From what I've read elsewhere on the topic of childhood myopia, the answer seems to be a combination of two factors:

1. spending a lot of times on screens means the eye only focusses at objects at a very close distance most of the time, this seems to cause eye strain which is why for adults working with screens the recommendation is to focus on objects in the distance every now and then

2. sunlight somehow (the exact mechanism seems to be unclear) contributes to the development of eye shape in children, in other words a lack of sunlight can cause the eye shape not to adjust as the body growth, impairing eyesight and causing myopia

The effect of 1 seems to be minute compared to 2 and the latter can be solved by literally just spending time outside. Putting your toddler outside to watch their iPad seems to be literally better for eye sight than taking the iPad away but keeping them inside. Of course the ideal solution would be having the kid outside without the iPad but if it's a choice between the two, getting them outside is the more important focus - and it's often the easier one too.