I'm the same height as you, and about 15kg over your weight - I've been told by many gyms & doctors that for my body type, I shouldn't go below around 65kg.

I think it's a lot healthier to work off your BMI fwiw, as the BMI range is very generous. I agree with MG, don't stress about your weight, you're a mum now and your body changes a fair bit after having a bubba!