In your first photo, sitting in the grass, the adult is centered in the frame. There's no way around that: the bigger adult being in the center of the frame is going to dominate and hold the viewers attention. In that photo, you can recompose/crop to make the adult not be centered to better balance the scene. Similarly, notice how much grass surrounds the adult (especially on the left edge of the frame) and how little grass is to the right of the child -- that has the effect of feeling like the child is pushed to the side, out of the frame. I bet if the adult was near the edge of the frame and if the child had more grass around her it would feel more balanced.
Shooting parallel to your subjects will have a tendency to make them appear uniform and equal.
In both photos the camera is square to the subjects which makes it easy to make size comparisons.
Square to the subjects means that the subjects are at the same distance to the camera and that the line that joins the 2 subjects is perpendicular to the line of sight. A way of changing that would be to photograph the child from behind the adult, at an angle, so that you don't really see the adults face (just cheek) and more of the child face (entire mouth, both eyes, etc).
Shooting more from the side can be a way to hide the similarities/differences in your subjects. As Vertigo notes, a wide angle lens can make it easier to highlight this distortion, which would help you de-emphasize the size difference.