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A quick update :). The car is in the garage because it would not start even after a jump start or hill start so they suspect its the engine. It is really frustrating because it happens regularly :(, I think its just an old car (it's a Citroën by the way).
Anyway, after dinner we sat the kids down and explained that we don't tolerate temper tantrums, especially from kids aged 5 and 8. We then told them that due to their tantrum, they are grounded for a week. In that week they have to tidy up their rooms, put toys back in their boxes etc and they MUST play nicely with each other with no arguing, fighting or name-calling otherwise the week grounding gets a week added until they learn to control their anger. I got a couple of sad/angry faces pulled but they realise we are serious so I hope it helps.