Is it a good idea to take a 6-year-old for trekking?

yes, my elder one went for her first trek when she was 6 and absolutely loved it :) she has gone for many more ever since, even though she is not really an outdoor person. do make sure that the trekking route is safe and okay for kids to go to. if it is too strenous, your kid may get put off from the concept of trekking. also, take your kid for trekking when the weather is favourable, not too hot or rainy or humid. have fun and take care :)