I have a broken coccxis (since i was a child, i m 39 now..) and little back pain and i will be doing skydiving (indoor) at the week end. is that safe?

Not unsafe: Coccygeal Trauma as a child should've healed long ago.The fact that you continue to have chronic pain is unfortunate but does occur in a small % of people who've had an injury to the coccyx. There is a risk for reinjury in the extreme sport of skydiving but standard precautions prevailing you should be fine. You could add a little extra padding to the tailbone for good measure.