How can I install an app on a family member's watch? Specifically, I would like to share some form of WhatsApp. But I cannot see how to install on the child's watch.

How can I install an app on a family member's watch? Specifically, I would like to share some form of WhatsApp. But I cannot see how to install on the child's watch. I have set up Family sharing and have designated all new purchases to be shared. Nothing works. This is a new Apple watch 6.