It's extremely difficult to explain constant pain to someone who has never experienced it. I did find one example, though, that seems to help. It's called the Clothespin Challenge. You can find the details here:
http://www.healthcentral.com/chronic-pain/c/5949/17583/clothespin
Other than that, just try to educate them on what is causing your pain and tell them what your doctor says. If you can find a way to help them visualize what has happened to your back, sometimes that helps. Many of us are visual learners and have difficulty understanding something until we can "see" it – mentally at least.
As for the pain medications, try to explain to them that when someone is in serious pain, pain medication doesn't make them high. At best, it makes them functional.
I'm thankful you have your two precious little girls to bring you joy and a reason to live. I wish you the best.