I don't think the guy was saying he was going to force his kid, against his will to be trader. My guess is the kid expressed interest. That said 15 min or so a day doesn't preclude other activities. In fact I would say doing multiple activities (trading, art, reading, games, sports, photography, etc.) 15-20 min per day each would be a great way to spend time with your kid and broaden their horizons. By providing experiences in multiple activities the child can better determine interests later in life based in his/her own experiences. A broader scope of experiences can only enhance that.
I have a daughter that I did this with. We have played soccer, softball, read, played video and board games, I taught her photography, woodworking, other arts and crafts, and much much more including options trading. We also enrolled her in dance and she has had creative writing classes. Now at 16 she has found of those she really loves photography the most and has won several first and second place awards at local 4H/Tri-Club shows. Now she wants to pursue that as a career is going to be an intern at a local photography studio this summer. She may have never found her passion for photography had I not invested the time to expose her to it when she was younger.