I was watching the gape intentionally. I have seen baby robins bring up pellets in the late afternoon. It always begins with this type of "yawn".
I shot non-stop, waiting. It happens in a split second. I was focused on the baby in foreground. Surpise of surprises. The camera caught it. My eye didn't see it. The baby in the back brought up a pellet. That's the fifth time I've seen this, always late afternoon. The pellet drops down into the nest. The parent robin will find it later while cleaning the nest and get rid of it, either eat it or take it elsewhere. Robin parents are fastidious "housekeepers". (That's another story.)
It's non-stop, all day long. Feed me! Open mouths. Both parents are tending to the nestlings which means the earlier kids are now self-sufficient. There's a pattern. Mama builds nest and incubates eggs. Both parents feed nestlings. Nestlings fledge and Papa feeds the fledglings. Mama builds next nest, incubates. Eggs hatch. Both parents feed new nestlings. Previous kids on their own. I did once see a older sibling come and examine the new nestlings. The parents permitted the intrusion.