Absolutely horrendous case and both the parents and hospital have some serious questions to answer.
The whole situation sounds like an accident waiting to happen if both patients and staff had complained about him running around unsupervised. The potential for harm for him and others was so high with the possibility of him escaping, being taken or having or causing an accident. The hospital had identified his lack of supervision as unsafe but it seems that they did little to actively manage the risk. Very sadly although an accident was entirely foreseeable, the accident that did happen was the worst outcome there could have been.
For a child of his age his behaviour is incredibly unusual. Most 6 year old children would know not to pick up a baby, let alone a poorly one they didn’t know in a hospital. Again, for most children of that age being told ‘no’ by an adult would have corrected them and prevented them from trying any more. Possibly he has some SN that prevent him from fully understanding risk. Possibly he has just been so poorly parented he has no idea of safe boundaries. Either way his parents have utterly failed to keep him and others safe by allowing him to run amok around the hospital.
Absolutely heartbreaking for the baby and her family.