For anyone who wants to do this with their kids, I would highly caution you to temper any expectations that they will be able to hear or talk to the crew: while entirely possible and also pretty easy, it's a very rare treat to make a contact. The crew keeps very busy and most do not carry amateur radio licenses.
The ISS is a trivial satellite target. I cannot overstate the ease of receiving it compared to other satellites. A $20 radio like a Baofeng UV5R or an RTLSDR dongle is all you need. A fancy directional antenna, worrying about doppler shift, tracking its position, etc. are not necessary with ISS to listen to the FM 145.800MHz voice or 145.825 (1200 baud AFSK ax.25) packet radio downlinks. Voice activity is incredibly rare as I mentioned, but packet activity is nearly constant as it consists of ground stations relaying through the ISS digipeater. If you want to be sure your setup is working, listen for packet data. You should also check online to make sure the digipeater is currently operational. For various reasons, it is sometimes offline.
Most of the ISS packet traffic is APRS and include position data which you can decode with a cell phone or a computer (or a radio that supports APRS or packet). It's fun to watch a visible pass while monitoring the traffic to see where it's all coming from. My kids who are typically not terribly interested in my nerd hobbies enjoy this particular one quite a lot.
In my experience the new voice repeater is quite busy on 437.800 output. While you won't be talking to astronauts, you'll be hearing it and often can see it move across the sky while listening. My kids get a huge kick out of it.
My son is fascinated with spaceships. He is addicted to the game Kerbal Space and follows everything about Space X, Nasa and other companies.
How can I stimulate his curiosity?
The first time I got to try using a telescope (the day a friend bought one), the very first object I saw was Saturn and seeing the rings with my own eyes was a very different experience than knowing they exist or having seen picture of them.
RTLSDR https://www.amazon.com/RTL-SDR-Blog-RTL2832U-Software-Define...
Use it to pick up weather satellites with the included antenna. Can spend ~$100 more to get images from the GOES satellites. Can use it to listen to ham radio, and local repeaters. Can use it during SSTV events to get images from the ISS https://ariss-sstv.blogspot.com/
Also micro satellites are becoming a thing. Strap a camera to a weather balloon, that kind of stuff.
