Penny Boards are a cheap option to get on a board with four wheels but are really not ideal for learning the ins and outs of Skateboarding (or Longboarding) as a beginner. While smaller lighter riders are better suited than adults for Penny Boards there are still better board alternatives in most cases.
The general issue is that
Penny Boards are so small that even kids and smaller riders have to use a very short stance (feet close together) just to fit both feet on the board — for maximum control and safety.
Stance = the distance between a riders front foot and back foot. A wider stance leads to more stability and control which is hugely important for beginners learning to ride.
Are Penny Boards good for smaller riders pushing & commuting?
The shorter Penny Board stance also makes pushing long distances (of a mile or more) much less comfortable - that means riders will probably end up leaving their Penny Board at home and will take their bike more often than not.
The same is ABSOLUTELY not true of Longboards which roll further with every push and are more comfortable to push in general — some riders have pushed thousands of miles on.
The short stance required by the 22 inch length of a Penny Board is also the reason that larger riders can look pretty awkward when standing with both feet on a Penny Board — that means kids will out grown Penny Boards more quickly than skateboards or longboards. Even the Penny Board Nickle (slight longer size at 27 inches) is shorter than the shortest longboard, and even shorter than almost all skateboards.
Longer Boards = More Stability, Control and Safety.
Contrast the short unbalanced stance of a Penny Board (feet close together) with the wider stance that you see on longer boards meant to make pushing, commuting and generally balancing and controlling your board easy. Longboards designed for kids tend to look like the longer boards bigger riders use, just smaller — that leads to better performance and more opportunities for kids to progress faster.
Penny Boards ARE however cheap. That is hugely important since riding ANY BOARD is always better than not riding (even a Penny which is not ideal for beginners). If you can't afford something more expensive get a Penny and start saving for your next board (a legit longboard or skateboard) that will let you expand beyond the simple stuff.
The last issue with Penny Board shapes is that a huge amount of the of the 22 inch total board size actually is eaten up by the nose of the board forward of the trucks (which you can't stand on while moving) and the tail of the board (which you can only stand on briefly due to the steep angle). Generally Penny Boards were designed that way to reduce costs by requiring less plastic which leads to a cheaper price — at the cost of riding platform and general comfort / performance.
If the trucks / mounting pattern were setup differently at least some of the instability that is built into every Penny Board would be mitigated — both the shortest Penny Board 22 inch original and Penny Board Nickle both experience this issue which makes the stance platform even shorter than it needs to be.
While Penny Boards are slightly better for riders who are smaller and lighter (vs larger riders) since the board is relatively longer for them, they are still a poor choice for kids to get started on since the tighter stance really does make learning to balance that much harder for a beginner.