Rusty didn’t have any fosters when he adopted Rex, and then waited to take on more until his new fluffy was fully healed.
Fosters are a new concept to Rex, since he came from a breeder and has never seen chirpies without a mother before.
“Wex, wook! Nyu fostew babehs!”
“Whewe am mummah?”
“…Daddeh and Spot am mummah!”
“…wut.”
Spot obviously doesn’t have the vocabulary to explain this, so Rusty helps him out and lets Rex meets their temporary baby siblings.
Spot will coach him on being a good big brother. There’s likely more art of the boys and chirpies to come.