The “Supine Vajrasana” is what I use after running or workouts that have stretched my quads hell and back. 30-60 seconds for an adult will do no damage. When I’m really sore, I only do one leg at a time.
If you were to sit on the floor playing with your matchbox cars, as an adult, you’d find your knees would start aching.
FTR: I used to sit like that and play ALL the time, and have not a single one of the negative issues described in that article. Kids have been sitting that way for years. It lets you play with your toys without your legs getting in the way. There is another “Ask the Doctor Website” that says “Sitting in this manner won’t cause any problem”. So, it seems to be one of those YMMV things.