> 6-7 years old, before then they'll get bored of them after 15-20mn since they can't read comfortably

You could teach them to read. My mother taught me to read when I was four before I went to school at five.

> in places that aren't adapted for children (e.g. in an airplane or train), a tablet can be a life saver. you keep them busy/playing for an hour or so, then give them their digital crack for an hour, rinse and repeat. audio books are a good compromise

Boredom is not fatal and in small doses is a good thing for creativity. Children survived and thrived perfectly well on long train journeys for a hundred and fifty years before digital crack.