First you need to remember that even though you want to use dark theme, you can also use lighter color. Thus, try to combine the dark color with any lighter color that the kid might like. The lighter color will work as the accent color that also compliment the darker color. This will also make the lighter color become more dramatic with the darker color as the background.
If you do not want to use vibrant color, then try to create a monochrome color scheme with several dark color with similar shades blend together. This will neutralize the dark color and make it appear friendlier and not overly dramatic for the kid to enjoy.
When it comes to dark color, that does not mean you can only use black as the dark color. You can actually use any color but try to find one which is on the darker side than the lighter or vibrant side. For example, you can use dark blue, dark red, dark green, and many others.
For those who are still in doubt by this color, you can start it little by little by using it on the wall instead of applying all at once to everything in the room. The dark color is very suitable to be used as the background for your interior decoration which make the other color pop up more. Then you can use any colors on the decorations as you please. Here are some of the dark themed kids’ rooms that you can use as example. Look at the color scheme and choose one that you like.
image source : pinterest.com