The part they got wrong: the children
Ask him what they misjudged and he does not hedge. The children.
His son was around twelve and in seventh grade, his daughter in fourth. Both had spent most of their lives in the US. The parents assumed they would adapt, because children adapt. What the parents had not modelled was how much smaller the tolerances are in an Indian classroom.
At the first school his daughter addressed a teacher by name, exactly as she would have in the US. It caused real offence. She said she was not going back, and they moved her. They were lucky with the second choice: a school founded by another US returnee, a scientist who had come back from Texas and built something closer to an American setup.
Then came the languages, which is the part that takes years rather than weeks. Tamil, Hindi and Sanskrit, on top of a curriculum. Online tutors for Hindi and Tamil, and his wife holding the rest together. His son is in tenth grade now and handwriting is still the hardest part. One household rule helped more than any tutor: no television Monday to Friday. The children go out, play, come back and pick up their books, because there is nothing else competing for the evening.
There were also the smaller cultural collisions that nobody writes into a relocation plan. His children called him "hey daddy", which his father disliked. His mother corrected his daughter for sitting cross-legged. None of it is dramatic on its own. All of it adds up, and his daughter told him more than once that she wanted to go back.
If school choice is the piece you are still working through, the CBSE, ICSE, IB and IGCSE guide for NRI children is the right place to start, because the board decides far more about the daily experience than the brochure suggests.