Well, here is the update..My budget wouldn't allow new heavy duty springs, because of all the other stuff I needed to buy to make it street legal.
But on the plus side, I did manage to afford an "add-a-leaf" kit..only 21 bucks. It has made a big difference in the weight capacity problem. Now when my kids are in the back the cart doesn't squat at all. And when they are not on the back, I still have a nice soft ride.
What a pain in the butt to put the spring kit on...I am sure I did much more work than was needed.
Thanks to all for the help and info!
If there is anyone on a tight budget like me, the add-a-leaf kit is a great alternative. I highly recommend them. Now we can all ride on the cart while saving up for a real set of HD springs.