I started childminding a year ago, from being a SAHM and for me it works brilliantly. I am a qualified TA but found that childminding suits my family life better as I don't need to use breakfast/after-school club for my own children. I would suggest you consider the following:
Would you work through the school holidays? As you are a teacher, your children are presumably used to having you at home in the holidays. (I work 3 days per week and term time only, and I am full....) If you only take on term time contracts then this clearly further limits your earning potential.
Don't underestimate the impact on your own children. Mine are 8 and 5. They cope, mainly happily, with having mindees back after school for 2 days per week, but I think they'd be fed up with any more often than that - also it curtails their after school activities and having their own friends round somewhat.
Could you cope without a huge amount of adult interaction in the day? Obviously you can go to toddler groups etc but it is not quite the same as having a staff room full of colleagues.
How many childminders currently service your children's school? Might be worth asking the office staff. My school had a distinct lack of childminders, especially as it is a 90 intake school and after school club spaces are hard to come by. Pretty much all of my (8) mindees are linked to school in some way. Either they have siblings at school, so I can pick up or drop off at school which suits many of my parents, or they are teachers' children. If there is a clear need then it will be much easier to fill your spaces.
Bear in mind too, that if you do have dc3 then they will count in your numbers and you will only be allowed 2 more under school age during the day, and not under 1 year old until your child has had their first birthday.
Good luck with your decision.