Awareness. That is the best piece of advice that I have for you, becoming aware of what you put in your mouth and restraining yourself from being a human garbage disposal of the leftovers that your little ones don't eat etc.
Being you are nursing, you are correct in that you probably can't totally restrict, but it doesn't mean you can't eat healthy. I believe the rule is 80/20. 80% of weight loss is attributed from what you eat, or don't eat. Ask your doctor what a safe caloric intake would be. Let him/her know you would like to lose weight as well and find out if 1800 calories is okay.
Make rules and plan to exercise 3 times a week for 30 minutes at a time. Once you realize how great you feel from it, you will want to do it more.
Best of luck! And here is a link to a BMR calculator. Knowledge is power!
http://www.internetfitness.com/calculators/bmr.htm