J.P.
Hey--
I may not give you the answer that you want, but here goes. I am a breastfeeding pregnant mother of a 3 1/2 year old. After much research I have found the following:
1. If you take the extra supplements your children will get plenty of calcium if you're getting plenty of calcium, nutrients, etc.
2. You can always give your older child a chewable calcium supplement if you feel it is necessary.
3. The human body produces exactly the right kind of milk for your child. While conventional wisdom pushes cows milk, it is perfect for a baby cow---they have four stomachs and digest things twice.
4. Children know what is best for them. There are more nutrients in breastmilk if it comes from Mom instead of from a bottle, sippy cup, etc. If your son is spitting out cows milk there's a good chance he's allergic to it or simply knows Mom's milk is better for him.
5. In the US we're a little weird about breastfeeding. We're the only country in the world that only recommends it for the first year. The World Health Organization says for AT LEAST the first two years. Psychologists agree that the kindest, gentlest way to wean is to let the child self-wean. World-wide the average age that a child will self-wean is between 3 and 4. After toddler hood a child does not need milk. Calcium, yes, but not milk. To self-wean you use the don't offer, don't refuse method for the most part. My very attached 3 year old has boundaries around when I allow him to nurse. Basically for a few minutes at nap time and bed time....and once in a while when he's really emotionally upset for comfort. I do refuse at times and he respects those boundaries because I've held firm to them.
6. You might want to call a La Leche League leader for support. It sounds like you may just want your body back and need some other mother's to talk to. You sound frustrated about being at home with your babies and this breast milk issue is more about that then anything else. Remember, you have the rest of your life to work. Your little ones are only little for a very short time. Try to enjoy them because soon they'll be off and running and you will have more freedom.
I'm guessing none of this is what you wanted to hear, but hopefully it will help you adjust to your current situation.