A used console in this price bracket would be very old and unreliable, so it's best to avoid them.
Luckily, you can build a very VERY decent gaming PC for 20k or less, if you go the pre-build way.
Since you're from Karachi, I'd suggest you to pick a Dell Precision T3500 workstation with a hexa-core Xeon CPU (X56** Series), 6GB of RAM (3 x 2GB Tri-channel) and 500GB disk drive for around ~10k. Then just buy an RX470 4GB with the remaining 11k.
This machine will be quite comparable to Playstation 4 Pro in terms of raw hardware muscle, take a look:
Plus, since it's a proper computer and not a console, your kids can also do their homework and other stuff on it whereas the console will be pretty much limited to gaming only.
Hope it helps.