Pick the paint color scheme for the treehouse. You can make it a miniature of your own home, using the same paint and trim colors, or you can pick a color scheme with your child. If she likes it, that's what matters, even if it's wild.
Paint the interior of the treehouse. Use a durable, flat latex paint that is scrubbable. For a fun switch, you could also use chalkboard paint which will dry to the texture of a chalkboard. Your children can then draw on the walls.
Paint the sides of the treehouse. Use a good, flat exterior latex paint. Paint the sides with a brush or a small roller. Depending on the type of wood used, you may need to add a second coat of paint. Let this paint dry for a few hours before doing the trim.
Paint the trim. Tape off the sides around the windows or door with painter's tape. Using a smaller brush, apply the trim paint.
Paint the roof. You can make the roof look like it has been shingled. Paint the base coat first, and let this dry. Then, using a small art brush, paint in "shingles" with a darker color of paint. These don't have to be perfect, but make general rectangle shapes the size of shingles.