Advice for a rookie coach please

Hello all -

This is my first year as assistant coach to a local girl's high school
volleyball team.  I love playing volleyball, but have never coached before.

The head coach had to have some surgery done on her knee, and unfortunately
her surgery date fell on the date of our first home game of the season (last
night).  So it was up to me to coach the team during this match.

We have 16 girls on the team.  8 of these have played previously, and the
others are new.  Of the new ones, 2 or 3 have shown some very good potential
and natural athletic ability and coordination.

During last night's match, we had some very long volleys.  We won the first
game, and then lost the next three to lose the match.  This took a full two
hours (so much for rally scoring speeding up the game!).  Being brand new at
this, I found myself a bit overwhelmed and stressed out with all the things
I needed to think about - making sure the girls were in their proper
rotation (after volleys lasting several minutes, sometimes they forgot if
they served, or if they needed to rotate), how many timeouts left, who to
sub in for whom, signalling our server about where to place the serve,
keeping track of weaknesses in our playing to point out during a timeout,
etc.  "Sensory overload" could be an understatement - I found myself
mentally exhausted about halfway through the third game.

I believe I made some mistakes in subbing.  The last couple of games we were
down by 8-10 points and the opposing team had 22-23 points.  So it was
apparent (in hindsight) that we were not likely to win this game.  But, I
was still only thinking about "there still might be a chance".  So I kept in
my starting lineup, just subbing a couple of the stronger players to give
one or two of the starters a break (2 hours of solid play is obviously
pretty tiring).  Looking back on it now, it seems clear to me that when we
were down by that much, that close to the end of the game, I should have
pulled several starters and put in some of the junior players, to give them
some playing time.  It just didn't occur to me.  That is, not until 2
minutes after the last game was over and one of their parents approached me
to ask, "So, what are the junior players on the team for?  Just to ride the
bus?"  I was a bit taken aback, and not even sure how to respond, and he
followed up with "I mean, they couldn't have done any worse".  I just told
him that any mistakes that were made in subbing could be chalked up to my
inexperience as a coach (in fact, this guy's daughter is one of the players
that has some great ability and we are working on her to be a setter).  We
parted on friendly terms, but it really got me to thinking about how I
should have done things differently.

So, my question to you guys is, what is your standard response (if there is
one), when a parent asks "Why didn't you play my child tonight?" (or, "Why
didn't my child play MORE tonight?".  Thanks for any insight.

Scott