For the picture you've described I'd try blurring the background unwanted person and all. Or try blurring it after you've cloned the person out. Your "bad paint job might not look so bad then.
To do this I'd duplicate the photo layer. Then on the top photo, use the magnetic lasso and select your son and his daughter. Right click and select
Invert selection then hit
Delete. It's not going to look like you did anything unless you hide the bottom unedited photo. Then you'll see that you extracted or cut the two of them out of the photo. Now make the bottome photo the active layer and select the edges of the photo (hold down the CTRL key while you click on the layer in the layer palette). Then click
Filter> Blur> Gaussian and try different amounts of blur to get a result you like.
HTH! Are you wanting to get in to digital scrapbooking or just fix some photos?