Why can't I forward SMS messages from Apple Watch to iPad when Apple Watch has a phone number?

All three of my kids have Apple Watches with cellular connections. We use those as their phones, and we've intentionally not purchased iPhones for them. (Ask me why later.) We also got all three of them iPads with WiFi. They don't have their own Macs or iPhones (yet).

What I'm struggling to understand is why they can get SMS messages on their Apple Watch, and yet the SMS message can't be seen on their iPad. How is this substantively any different from a person with an iPhone and an iPad, who can do that? It should be the same, and behave the same way.