How and where to get started with a 5 year old around here?

Catbelly,
 
Welcome to the site.  I used to fish Duck Hollow a good bit, I lived in Greenfield for a few years, then the South Side.  It is not the best place at all for a 5 year old.  The parking area is on a "pier" (old barge tie off) that is about 20' or so up.  You would have to watch your son like a hawk.  Access down to the water isn't the best either.  The worst part of the spot and of most spots in the city limits and even a little out, are the combined sewage overflows.  Any sort of rain over a drizzle ends up dumping raw sewage just up stream of Duck Hollow and turns it into a mess.  I watched guys wading out there trying to make there way back in after those things opened up.  One of the most disgusting things that I have ever witnessed.  That said, if you avoid the place days that it is going to rain and let the rivers clear out the waste after 2 days or so, fish definitely hang out there.  Guys catch some nice fish from there and when he is a bit older, might be into hanging out with dad on an evening doing some cat fishing.  For me, it was a place to fish if I had an hour or two to burn.  Anything more than that and I went to something a bit cleaner.
 
This time of year is rough for catching fish.  Many fish are heading deeper and then suspending over specific structure and only biting when they want.  So it is hard to get them from shore and not very easy to get them from a boat either.  If you had access to a farm pond, but far the best choice!  If not an option, then I would specifically target bluegills with him.  Chunk of crawler, meal worm or a small plastic (1" Gulp minnow) always seems to get some bites.
 
This year, with coronavirus, most public spots are packed with people.  Especially those spots that are easy access, which I would assume are going to have the fish a bit smarter.  When my kids were that age, I found the lakes with playgrounds near by always made for a good option.  Once bored, which if fish aren't cooperating, then it is easy to get them back to a happy place.  Maybe, but don't force it, they will want to fish again.
 
Under normal years, North Park Lake is about perfect for taking a 5 year old (unless you have access to a farm pond).  Lake Arthur/Moraine State Park is another.  But both are pretty well packed on weekdays, like it is the new Saturday.  Cannonsburg Lake has bluegills that are usually willing but doesn't have a playground, same with Peters Lake.  I would assume it is busier than normal, but not as packed as those others, but just assuming.  Cross Creek, Loyalhanna Dam, Keystone Lake, Raccoon Lake, Yellow Creek Lake are all about an hour from you.  A few of those have the playground thing too.  Most are going to be slow fishing at this time of year.  In the fall and spring, the catching is a good bit easier from shore.  
 
There are a lot of creeks around that hold small mouth bass, trout, creek chubs and other larger fish as well.  With a 5 year old, they are a bit harder to fish.  Much easier with someone that is able to safely wade the creeks.  But there are some holes that have easy shore access.  Check out Google Maps satellite view and should be able to find some.  If you see some that are smaller, but have deeper holes, it can give you the added entertainment of catching crayfish, etc.  Or Google some of their names and "fishing" and likely find some videos or chat about them and likely find the location.  This is about the same said with the other rivers in the area.  Some great places on the Allegheny, Yough, Ohio, Kiski and even Mon, but most you would up your catching a bit by wading or floating.  Definitely avoid with high water with a 5 year old.  It is also a bit harder to fish moving water with a small kid.  With a lot of keeping line tight, getting hung up on the bottom and casting over and over.  These would be something I would graduate him into personally.  Obviously, if you live right near a creek/river or have a camp on one and fish it all of the time, those younger kids will catch on.  But still water is much easier.

catbelly, i've got 16 and 10 year old sons that have been fishing their entire lives in the pgh area, although we are north in butler county.  
 
re read porktown's post, because it is spot on in regards to this time of year for the type of fishing you'd be looking at with your son. 
 
i would assume like most 5 year olds, he'd be pretty happy catching bluegills or other panfish, so that's probably your best bet.  i'm not at all familiar with places to fish with him 5 or 10 minutes from where you're at, but all of those places porktown mentioned should be fairly easy to find something fun for him to catch with a good old bobber and worm.  6# mono line, a size 8 or 10 hook and a bobber a foot or two above the hook should get you into plenty of them.  
 
i almost forgot, glade run lake near the butler/allegheny county border might be a really good place to check out too.  it was drained and refilled a couple years ago, but stocked heavily with bass, bluegills, catfish and crappie.  the bass there aren't real big yet, but they are numerous.  
 
right now you can catch as many bluegills as you want in some places at lake arthur, and i would assume most of those other lakes.  hit shallow, weedy areas with mud or soft bottoms in the morning or evening.  if you want more specific info on where you could find some of those at lake arthur, let me know.  a bit of a haul for you probably but plenty of places there to take a kid to catch bluegills right now, and maybe a bass, perch or crappie too.  
 
i would also guess that being new to the area, you probably don't know a whole lot of people yet.  ask around at work or other places where you're getting to know people if anybody can take you to a farm pond or lake.  in my opinion, nothing beats some good old farm pond bluegills and bass (and whatever else someone might have thrown in there) for a 5 year old.