Birthday-Christmas collision does indeed sound like grounds for ALL THE PIES. :)

I'm pretty sure one could use something other than tapioca as the thickening mechanism; I like tapioca, even in fish-eyes-in-glue form, and it's reliable about setting fruit juice for relatively small volumes of thickener, but I can't see why cornstarch wouldn't work. It would just need to be rather more cornstarch for an equivalently firm consistency.

Putting tapioca in yoghurt seems bizarrely odd. I suspect them of yoghurt that won't consistently yogh and they have to cheat.