OP:
"...one of my strobes fail on me during photo shoot. .
.. there's a picture that the client really wants.. any idea how i can fix it? like what techniques>?..."
To be honest, John, for a pro shoot, you appear to be using a dreadfully inadequate, makeshift setup. For example, in addition to the missing front light (fill? key?) that you mentioned, you are also missing at least two more groups of lights that are commonly used in this sort of shot. Specifically, I see no evidence you were using any lights to blow out the background, and no evidence for hair/rim light(s).
Their absence explains why the background is such a mess and why there is no detail in their hair. Also, because the group is standing only a foot or two away from the seamless, my guess is that you are attempting to work in in an area that is too small to accommodate a group of this size.
In addition to the "missing" lights, it appears that there is a very odd, strong, fairly large area light source to the (viewer's) right of the group. The position of this source -- too low and at too far to the side -- is what is causing the monster lighting effect. Look at the shadows and bright areas on the face and shoulder of the (viewer's) right-most girl. Was there a window in that area?
I am not pointing out these deficiencies to be critical of you. Lord knows, I have shot in many makeshift studios myself. I am pointing them out only to give a sense of the magnitude of the lighting problem that you are trying to fix after-the-fact, in post.
Because of the factors mentioned above, my take on post processing is similar to what the other responders have already said: *It will be difficult*. To do it right, you need to correct for all the shortcomings mentioned above, as well as some that I did not mention. Your largest task is probably that you will need to add light to a large number of small shadow areas. These almost certainly need to be lightened manually and individually. If one tries to fill them in by global controls, you are going to see adverse effects elsewhere in the image, such as in my example image, below.
My final comment is that in a pro family shoot, the photographer always presents several images to the family for them to select from. This means that any PP efx that you apply to one image, you have to apply to all the others. This is an absolutely staggering amount of work which is far from guaranteed to produce an acceptable result. If it were me, I would fess up to them, explain the problems, and offer a free re-shoot. If you have a question about how one makes do with limited equipment, I'm sure some of the folks here will be happy to offer suggestions.
Below is my quick attempt to partially correct some of the lighting problems using only global controls. All the changes you see were done using one pass through ACR followed by one pass through Topaz Adjust. IMHO, it still is far from a pro-level shot, but hopefully, added to the previous responses, illustrates that best that can be done without spending crazy amounts of time adjusting the lighting shadow-by-shadow in each image.
HTH,
Tom
PS - I'm sure you have already noticed, but always be on the lookout for compositional and posing issues such as the hands-from-hell such as in the bottom left corner of this image, the giant protruding legs, etc.. ;-)
