"He asked if he could watch Dr Who. DH said no. DS1 asked why. We had already told him but he kept on so I said I didn't have to justify our decisions and he said I did."
well, you DID justify your decision if you had already told him why you didn't want him to watch it
i think saying "no you can't watch Dr Who because i say so and i will not justify my decision" to a 9 yr old would be odd and unfair
but you didn't do that
what you are saying, i think, is that you will not be drawn into endless discussions over subjects you have already made a firm decision about, and explained why
i think that sounds perfectly fine
with the only reservation being, the more children are allowed to negotiate and the more they see us being flexible, the better their negotiating skills and ability to be flexible will be
but everyone has a point at which they need to say "enough, we have discussed this already"
FWIW i think parenting courses can be great
they give you skills and a chance to practise them, confidence in what you are already doing well, plus a chance to let off steam about difficulties, and hopefully support from other parents in the same boat
good luck with it