Well, I thought I would try to help you with your photo. It is very dark and I thought it could use some work. I was not able to modify the parts you did yourself, but the overall tones are better here I think. I also sharpened it a touch.
If you send me your photo larger, I can do the same to it and send it back -
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There is a lot of ambiguity around the boy - how he is leaning against the wall makes him look off balance. You will have to handle that with some delicacy if you want to pull it off. The knee and draping hand on the girl are larger than they would be in life - photo distortion. I would reduce those parts about 2-3% and composite the image to reduce the problem.
I think that if this were a commission, then you might want to go for something a little less casual in dress and pose. However, since these are your kids, you can dress and pose them as you think will suit the painting and your lifestyle.
If you have the time, set them up again and try a couple more poses - you might come up with a better shot.
Hope that helps!