Then I seized the moment to introduce them to watercolor pencils. They are water soluble and fabulously versatile. When you hit the lines with water, they can be softened and blended to mix with the colors of the painting.
You can dip them in water and draw with the wet line. This produces rich color. The lines flow. Off they went drawing tree branches.
Trouble painting thin lines? Draw in the color. Easier to handle for some- and you can vary the line weight between strong and faint. They mix well, too.
You can draw on wet or dry paper, color on color, add in details, or hide your outlines. Try them - you just might like them. I suggest getting two or three to test brands and colors before buying whole set. There are certainly some colors - more neutral - I use more than others. Enjoy!