I like your Rubber Bones experiment. I never heard of it before, but I googled it and found out how to do it.

I like your milk experiment too! I'm surprised the bones were softer, but I'm not surprised that the bones didn't get harder. Animals make bones in complex ways, with cells called osteoblasts that build bone and cells called osteoclasts that break bone down. Those bone cells would all be dead after the bone was cooked (I'm guessing the bone was cooked and then the chicken was eaten off). Or, soaking in vinegar would kill the bone cells, if they weren't dead already. And, the bone cells probably don't live a long time after the chicken dies, because they're not getting food any longer; and food is needed for cells to live and grow.

I wonder why the bones got softer.
1. Do you think more calcium soaked out of the bones while the bones were in milk? This might have happened even though there is calcium in milk, because there weren't any live bone cells to build calcium back into the bone.
2. Do you think the bones got softer just because they were sitting around in some liquid, where they were decomposing; or do you think milk made the bones get soft faster than they would have in another liquid? At first I just thought of milk vs water, but then I thought of vegetable oil, which is very different from water and milk. Bacteria or germs might decompose the bones in milk or water, but bacteria or germs wouldn't be able to live in oil, because all living things need water. That's one of the differences between milk or water and oil.
3. How could you measure the softness of the bones? I think I might bend the bones and put them next to a ruler and take pictures of them with my cell phone camera.

Finally, here's what I found on the internet about Rubber Bones, from Science Bob:
rubberbone
The project above is a DEMONSTRATION. To make it a true experiment, you can try to answer these questions:
1. Does the length of time the bone is in vinegar affect how much the bone bends?
2. Do smaller size bones become "bendy" sooner?
3. Do different types of vinegar affect how bendy bones become ?
Keep asking questions!