A printed backdrop is flat. That sounds obvious, and it is exactly why it photographs the way it does.
What depth does to an image
When a camera looks at a real three-dimensional scene, things at different distances fall out of focus at different rates. Foreground leaves blur one way, the child stays sharp, the background dissolves. Your eye reads that instantly as a place.
A printed backdrop is all at one distance. Everything on it is equally sharp or equally soft, and the result looks like what it is: a child in front of a picture. No amount of editing puts depth back in afterwards.
What a built set involves
Three layers, roughly. Something in front of the camera to shoot through, a mid-ground the child actually sits inside, and a background far enough back to fall away. The foreground layer is the one most studios skip because it is fiddly to light, and it is the one that does most of the work.
Building that takes hours, which is why it happens the day before a session rather than on the morning. A set assembled while a family waits in reception looks exactly like a set assembled while a family waits in reception.
How to tell from a portfolio
Look at the edges of the frame. In a built set there is usually something slightly out of focus at the border — a leaf, a fabric fold, a prop. Look at the floor, too: a printed backdrop usually curves up behind the child with a visible transition, whereas a built set has a real floor treatment that matches the theme.
Then look at ten images from the same studio. If the same three backgrounds recur with different bunting attached, you know what you are getting.
When a backdrop is the right answer
Honestly, sometimes it is. For a clean studio portrait where the child is the entire subject — a portfolio headshot, a simple milestone frame — a plain seamless background is better than a set, because there is nothing to distract from the face. We use one regularly. The argument here is not that backdrops are bad; it is that a printed jungle is not a jungle.