In addition to this great Mumsnet board there is www.NannyJob.co.uk which has a message board where you can chat with other nannies about working as a nanny.
Other things to consider:
As a teacher you may be part of a pension scheme. By leaving teaching your pension will be frozen at it's current point. Talk to your pension provider regarding what happens with this pension. Also you need to opt back in to the state pension if you are no longer paying into your occupational pension.
As a nanny you won't have a pension... so you need to set one up yourself or be very good about putting aside a portion of your salary into a savings account.
Future employment prospects:
Once you leave teaching, you are unlikely to return as your knowledge will out-date. As a teacher you have the ability to work your way up through different levels. Thus if you worked your way up, your future earnings could be quite a bit more than your current earnings.
As a nanny there isn't any career progression. I have no idea what I will do when I get too old to be a nanny - probably working for my local council doing childcare advice. That could be a salary drop.
When your son is school age, your working days will fit a lot better with his days at school. You will probably need to use a childminder before/after school but not for many hours. So your childcare costs will not be a large percentage of your salary.
Try to think about what happens in 5 years time, 10 years time etc.
When mothers go back to work after maternity leave they often don't make any money after paying for childcare. So if you have done the calculations and make SOME money, then that actually sounds quite good. As your son gets older, then childcare cost could drop, thus meaning you make more money, plus you could get paid more in your job.
Consider your return to work options... you may not need to return full-time, you may be able to job-share with a colleague. This would then keep you intouch with work, keep the pension benefit and your place on the career ladder (you may even go up it a little, depending how it works as you may get credit for long service record).
Remember that your partner has responsibilities for his child. He can do some childcare himself at times - perhaps he could work 4-days per week, meaning that you need one less day of childcare. Grandparents may also be willing to help out.