The short answer.....
You don't. Graphic design is about balance, proximity, white space, interaction, etc. These are things a child is still developing a sense for. You can't push a child to interpret their world around them faster than they can develop.
The longer answer....
While painting and drawing is one aspect of some designer toolboxes, it is not the be-all-end-all. There are and have been very successful designers that have never "drawn" any representational objects. Design isn't about a tree looking like a tree. It's about where that tree is and how it relates to other objects around it. The flow of the eye across a page/piece. The inherent balance conveyed when looking at a design. All things children are still learning well into their teenage years. It's entirely possible to paint/draw like a wizard with miraculous skills, and yet, never be able to design. Just as the inverse is true -- have a great eye for design, but not be able to draw/paint.
Painting/drawing techniques can certainly aide in removing limitations when creating a design. If you can draw a tree, you don't need to go find that specific angle of a tree as a stock image to use. And through painting/drawing children can start to get a handle on things like proportion, scale, and placement. So there is benefit to those skills.
In terms of tools... you can paint and draw in Photoshop or Illustrator. If the child has an interest, let them use professional level tools to explore their art. Adobe comes with a hefty price tag.... so you can at least use software which is relatable to design, such as Inkscape or The Gimp. That way they'll be a leg up on the tools when they do gain a better grasp of design concepts. Be aware though having the tools is not encouragement to "design". Software is software. Design has a rich and lustrous history long before any software entered anyone's mind. Focusing on tools will do nothing more than foster an interest in the tools, not design. Design is not Photoshop, Illustrator, Adobe, Gimp, etc. In fact design has nothing to do with the tools. The tools are about production.
I, personally, don't think you can "get a child interested" in anything. They either are interested or not. And you can't force an interest. If they like to paint and draw, support that all you can. If they continue to improve and do things such as start adding text or creating more "layout" style artwork, then you can explore topics such as letterforms and typefaces or balance and whitespace.
I suppose you could encourage them to make signs, flier, posters, etc. Things that are more "design" oriented as opposed to painting/drawing.
I don't think a tablet is necessary by any means. In fact, in terms of "design" a tablet may be a hinderance, offering too much freedom to draw/paint as opposed to focusing on things like placement.Tablets are not a tool every designer likes or uses and is not really mandatory in any sense. (Disclosure, I use a tablet and haven't touched a mouse in over a decade, other than to install tablet drivers on a new system).
In the end, all you can really do is provide the resources and see if it catches on. As I'm sure you're aware, an interest at 10... may be a distant memory at 12.