Yeah, I agree, as a short-term solution (i.e until hen and chicks are no longer there/together) just chuck in some more stuff, a good THICK thickness of it. You can put it on half of the pen, then wait til hen and chicks have moved to that high ground and then put the remainder on the other still-muddy half of the pen.
Of course, when the chick-raising is over and the hen can be confined/moved/handled/whatever, you will have a substantial big job cleaning the pen OUT but you gotta do it or it'll be even worse next time.
Then, go take a look at my "fixing a muddy run" page (link in .sig below) for a bunch of things you can do -- preferably do as many of them as possible -- to prevent it getting muddy in future. In fact you might want to take a look at that now b/c it is possible that some could be done NOW to minimize how much worse it gets while the hen and chicks are still in there (e.g. trenching and gutters and extended downspouts)