Teaching a two-handed backhand as default is best practice these days, especially with children whose game is most limited by their physical strength. And in group lessons with large coach:student ratios trying to teach two different strokes adds additional overhead - ie more coach time talking and less students hitting.

However, a good coach will look for the dominance between hands. A good two-handed backend, especially the straighter top arm style (driven in line with the shoulders, the more common style in the mens game) is extremely dominant in the top arm. Kids who struggle coordinating that balance of power correctly and pull hard with their bottom arm often struggle to learn a reliable two-handed backend (I was one of them until i changed to a one-hander as a young adult). Those students i would have experiment with one-handed backends.

The problem is, so few coaches my age and younger have one-handed backends they don't know how to teach one.

(I was a tennis coach for 15 years before moving into tech)