An alarm clock and a timetable would cost you about £20 maximum, you'd be on time for school and your children would learn about organising themselves from you.
You just need a routine so that you can work through it in your mind and work out what comes next.
I found it helpful at first to have my "leave the house" time set, then work backwards. The "leave the house" time needs to be set at a time that allows you to get to school, park, be ready in the playground and have 10 minutes to spare for the inevitable lost car keys/reading book/wet pants/pooey nappy.
So for me:
School starts at 9.00. Children line up at 08.55.
The school journey is 6 minutes by car, plus a 3 minute walk. So I need 6 + 3 + 10 minutes =19 minutes. If I don't leave the house by 08.36, I'm in danger of being late.
But I don't tell my children that. They think that we're 'late' if they're not ready by 08.30. They've sussed out, now, that we have 3 'lates': 'late' means 'we won't be able to play in the playground before we line up'. 'Actually late' means 'we'll only just catch the lining up/we will have to go straight into class without lining up'. 'Really late' has only happened twice and means they have to go in through the office entrance, but are there before the register is called.
Then you just work it back. 5 minutes to brush teeth. 15 minutes for getting/eating/clearing away breakfast. 15 minutes for getting dressed. 20 minutes for your DS to have a bath/dry himself. 10 minutes for hair brushing/coat on/bag ready.
So if school starts at 9, you need 20 minutes to get there, etc., you know that if your DS is getting dressed after 8.05, you're late. Because all the steps after that take 35 minutes and you need to leave at 8.40.