Deep gray and all the other accents in silvery old wood and cream. This will add drama, allow the other elements to bring ion light and set off the concrete wall as a true feature. If you want light instead of drama, then go for a very pale beige stone. It will echo stone colours in both the concrete and rocks...

Now for Mr 16 months... I really can't decide. His room is going to be on the cold side of the house, with not so much sun so I want it to get that cozy sort of feeling when you come into it. I was thinking yellow or orange curtains? But then I don't know what colour to do the walls. These are some fabrics I like:

Photos would help us a lot but mint and grey usually goes well with charred oak.

OK - I'm a beginner here on Houzz so my opinion isn't worth a lot but here it is: the room is a bit bland and lacking in character. I think you have to find some pieces you really love to bring it alive. All the suggestions are good for widening your horizons, but ultimately, you need to find things that matter to you, because it's your home and not a magazine photoshoot. That said, if it were me, I'd throw a couple of small, colorful rugs down, and put either one large painting or three smaller ones over the bed - not in a line but at different heights - to break up the binary look. I'd also put something on the wall above the heater. Then, play around with a few cushions and maybe a quilt that picks up some of the colours in your artwork.