We haven't seen these guys in so long!!
So here's to help everyone: Agnes' and Henny's first appearence was all the way back in Book 1, Chapter 2! As for Tia and Sammy, you'll want to look at Jackson's and Olga's early MiMos, specifically Nuovo Terreno ;D
This is actually Sammy's first illustrated appearence: kudos to Yamino for the design!