messaging with apple watch and an ipad

My child has an apple watch that I just set up with my phone. So she now has a phone number just for the apple watch (not a real phone number). She can receive messages on her watch through that phone number OR her apple ID.

But then when she goes onto her iPad, she checks messages and half of the messages aren't there, because they've gone to her watch phone number, and not her apple ID.

This seems crazy to me - I'm sure I'm not the only one with a kid who isn't old enough for a phone, so they have a watch. And they have an iPad also. If it's all linked to her apple ID (including the watch phone number), then why would they also not show up on her ipad?