The reason it doesn’t work is because ‘discipline’ isn’t just you saying stuff in huff. No one would give a shit about that if they listened to it day in day out. Kids aren’t stupid.
You cut the verbal negativity. You only praise. Everything they do right (even if they’re not doing much at all!) you praise, smile, hug, kiss. You basically become Mary Poppins. You also instruct and coach in a very positive, affirming way. “Ok sweety, just pop that yoghurt pot in the recycling… good girl’ etc.
You have to be calm and in-control even if you don’t feel it.
When they misbehave, or don’t do as you’ve asked - you become very disappointed mummy - stern face, no eye contact, no shouting. Ignore the misbehaving child and if appropriate highlight the good behaviour of the other child. “well Jack had been a VERY good boy haven’t you recycling your yoghurt pot - do you want to go and get your bag and we’ll read your school book… good boy.” Quite often this may lead to…” oh look, polly is putting her yoghurt pot away now - good girl - go and get your book and we’ll read your’s after Jack’s” - so the second they are ‘good’ you switch back to positive verbal affirmation.
So no judgment, no shouting, no naughty step (pointless) - just pure Pavlovian psychology - attention for good behaviour and little to no attention for poor behaviour.
If they are kicking off or doing something dangerous - you can still employ this by removing them physically (gently) but no eye contact, calmly, no drama, no words. Once they are regulated again you can communicate verbally in a loving and positive way.
Do this consistently and instinctively and they soon learn which behaviour gets them smiley, happy, sunshine mummy and which does not.