There's another possibility as well. One that I'm leaning towards...
It's possible that Jess and her baby were asleep, the night before Gus was reported missing.
. And something bad happened to little Gus that night---possibly Josie lost her temper and accidentally punished Gus too severely? Or an accident of some kind maybe?
And so the GP'S, or maybe just Josie, disposed of the evidence and the child that night...maybe using the motorbike?
Then early the next morning Jess fed her baby, and she probably thought Gus was still peacefully sleeping? And Josie said "let's get an early start on the furthest fields today---and go mend those outer fences."
And so Jess had no idea. She expected to be back that afternoon to see her 4 yr old for supper.