BBC article - not yet stated if the driver was under the influence, just that she was arrested on suspicion and has been bailed until the New Year. However some newspapers say that she was not found to have been drinking/taking drugs. I guess all that is yet to be decided.
So the only thing we know for certain is that the children were illegally driving quad bikes on a public road, with their parents permission and knowledge.
Poor child Poor family Poor driver
BUT the point remains that the children were illegally on those bikes in that location and should never have been there in the first place. And had they not been there the accident would not have occured.
The parents will know this and will have to live with this fact every day from there on in. They will know that they judged wrongly and that others will judge them accordingly too. They will have a lot to live with, regardless of whether they are criminally charged are not.
Should they be charged? I don't know to be honest. The driver may wish to press for charges possibly - depends on damage/injuries they may have sustained as a result (if found to not be driniking/drugs). Some may want them charged as an example to others. I don't think going to prison would be the answer, but they do need to know that they did wrong and take responsibility for their own misjudgement in this case - however, I suspect they already do, how wouldn't?