When doing your stretching, you should do a sustained stretch, holding the stretched position for a slow count of 10. Do not bounce while doing your stretch. If it is not painful, it may just be the fluid moving around in your hips. Sometimes grinding and popping is evidence of arthritis. If this is a problem for you, I would talk to your doctor about this.
Children definitely recover more quickly than adults from hip and knee fractures.
Children recover more quickly than adults from any injury. Hip displacia does happen in humans. It is a problem of childhood. It can be fixed if it is identified early. Most pediatricians check for this in all newborns.