1. Used the largest image you have on Flickr.
2. Cropped it.
3. Enlarged it to 200% with Perfect Resize 7.
4. Cleaned up the yard with spot healing brush and patch tools.
5. Sharpened with Smart Sharpen.
6. De-sharpened some areas that got funky by applying a mask and drilling through it.
7. Applied a vignette.
It is a wonderful snapshot of two lovely children. And even better that it means so much to you.
To make it a great picture photographically speaking may not be possible though. It would have been nice in portrait (vs landscape) either including the children's whole bodies (including feet) or framed in very close on just their upper bodies. Unfortunately you cannot now crop it either way because a) their feet have been cut off and b) they are too small in the frame. Cropping in enough to even get rid of the background junk will throw away at least half your pixels.
I've taken a stab at it anyway - cropped in, cloned out the little bit of backround stuff that was left, cloned the boy's shirt a little longer so that his undies aren't showing and made a few minor brightness and color adjustments in photoshop. I think you can see though that this leaves you with a less than stellar resolution.