white background

If the photos have already been corrected, one clean way to do it would be to simply create a Subject or People mask for the kids in one step, invert the mask, and increase the mask’s Whites value until all gray in the background gets clipped to white, as shown in the demo below. If the maximum Whites value isn’t enough to remove all the light gray, then also increase the Highlights value.

Note that seeing the difference is easier if the Background Color (the area outside the image) is set to White, as shown at the beginning of the demo. To change the Background Color, right-click the background. (If right-click isn’t set up on your Mac, then you can Control-click the background instead.)

Because the Subject mask is an AI mask, you should be able to set up the mask only once and then use Copy Settings and Paste Settings to transfer that mask to all other images with a white background from the same shoot. The AI mask will automatically recalculate the subject mask for each image so that the only adjustment you might have to do is slightly tweak the Whites and Highlights options. But there’s a high chance you might not have to change anything, just set up the AI mask once and then copy and paste it to all the other images in one step.

When you say "came out differently"- what are you referring to specifically?- Exposure?, White Balance?

If 'Exposure' differs then you may be able to correct by selecting all images with the best one as the 'most selected', then-

MENU: Photo > Develop Settings > Match total exposures   [Ctrl+Alt+Shift+M]