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thanks for your answers.
The dogs have both been trained + are usually well behaved. We had a behaviourists session and she says its because they are frightened of the bikes, therefore they woof and get offensive which results in the bikes riding away so therefoe next time they think "aha! i'll woof and the bike will go away"
They are usually well behaved, but i really think there is no training them into not woofing,
The childrens parents have access t their garage, and even if they didn't it somehow dosent feel right to ask them to stop riding their bikes - it's what kids do, and makes me feel like a grumpy old woman, just a single step away from shouting "GAH" and shaking my fist at them