i would say you wake up at least with an extra margin of an hour, and wake them up at such a time that you have at least 30 minutes extra to spare.
for instance, my kids, one a kindergarten student and the other a primary school kid, have to leave at 7:15. so i wake up at 5:30 and wake them up around 6:15. by the time they wake up and take their sweet time and are ready, we can make it to the bus with ease.
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