Dear Vianna,
Your daughter is the Girl with the Pearl. Lovely angle, lighting, excellent resource. I also like the picture of your son. Nonetheless, the lighting is superior on your daughter, and I think that's what you should seek with your son so that they are compatible.
The things that I think count most in painting a pair of portraits are:
canvas size
head size
light color and quality
background continuity
and as John mentions, a relationship, direction wise.
So deductively, that leaves you with working with your son's image. Here are some thoughts:
If you are working in photoshop ( I have CS, don't know much about the other versions,) you can adjust color temp through Image, Adjust, Photo filter. I don't know if that will be enough to equalize the color temp, you'd have to try it to see.
Directionally speaking, you could flip one of the images to have the kids face each other. However, the light directions will be opposing. You will have to decide if that works for you or not.
Or you could work both paintings simultaneously, using a color photo image of your daughter , and a high res B/W of your son. Make the color quality and harmony in his painting agree with hers.
Of course, you can always take more pics of your son
Set him up with the light and direction opposite to your daughter. It might be a good time to experiment with different white balance points in your camera. (Or Diane can come by with her fancy camera, and you can work on the lighting together.)
Good luck. I know these will be excellent paintings.