Tom is right... these photos will never look right for 2 reasons.
1. They were not shot on a white background, so cutting them out and pasting them into one removes things that SHOULD be there.. like shadows. Even if the baby was lying on a white floor on a nice smooth white scoop, there would be shadows under the baby where it makes contact with whatever it is lying on.. see below.
On yours, there's nothing.... so obviously he looks cut out and pasted in.
2: Your lighting.. you're effectively lighting that baby from below. He/or she (I never can tell with babies) is lying down, but you are lighting it from the left... so in effect, the light is hitting the baby's face from below... giving that torch under the chin, spooky, let's tell ghost stories around the camp fire look. If I rotate your shot you'll see what I mean...
There are shadows ABOVE the nose, and ABOVE the cheeks... you've lit him from below. If a baby is lying down you need to think in terms of rotating everything 90 degrees from how you would normally do it.
So in short, you need to work on your lighting, and if you want a white backdrop, you need to shoot on one.
The biggest mistake with shooting on a white backdrop though, is that people always put the subject very close to it, and that causes all manner of problems because the light(s) that are lighting the main subject, are also lighting the backdrop, and hence you'll get shadows on the backdrop... and then you're back in the same boat, spending ages in post trying to remove the shadows and "whiten" your backdrop. You need distance between subject and backdrop.
I don't often shoot on a white backdrop, but when I do, separating the subject from the backdrop is the only way to get a pure white backdrop straight off camera.
Here's a shot of mine.. utterly un-retouched and unedited... not even levels, curves.. nothing.... straight from camera.
Here's how it's lit as a top down view.
You need to light backgrounds separately if you want them purely white. It's also a myth that you have to expose them 2 stops more than your main subject. The white background in the above shot is lit to the same aperture as the main exposure. If you are correctly using a hand held meter, the correct exposure will render white as white, or close enough to it as to make clean up easy. Go much over, and your background becomes a light source, and you'll get light wrapping around your subject from the side/ from behind.
So with kids moving around on a studio floor... you'd so the same thing., but if they are ACTUALLY on the studio floor you won't need to cut them out, and the shadows under their feet or hands or whatever will be genuine.
I'm redoing the shoot again so hopefully second time lucky, the baby was actually lit from both sides but for some reason one lamp was brighter than the other even tho they were on the same settings and at the same distance.
