Pep can I suggest trying to shoot with natural light, if necessary bring them closer to a window or door. If you want to use artificial lighting then I'd suggest bringing bringing the subjects/children further away from the wall so you don't get the harsh shadows. Also if you wanted to unify the image try to get them looking in the same direction or concentrating on the same thing, playing together or even looking into the camera. Also see if you can use the rule of thirds for balance.
I really hope this is what you were after as a critique and apologise if I have offended.
Kind regards, Nicole
Thanks, Nicole for your critique. Of course, you have not offended me at all and you don't have to apoligise. In fact, I know this is not a conventional portrait and I wanted not to follow any stablished rule. I assume that result is not as good as it could be. The only thing I regret is taking the photo so close to the wall.