It's not really feasible that Mirri Maaz Duur took the child without anyone knowing. Although she delivered the baby, she wasn't alone in the tent; Danaerys' hand-maidens were also in there.
“He never lived, my princess. The women say He faltered, and Dany saw
how the flesh hung loose on him, and the way he limped when he moved.
“Tell me. Tell me what the women say.”
He turned his face away. His eyes were haunted. “They say the child
She waited, but Ser Jorah could not say it. His face grew dark with
shame. He looked half a corpse himself.
“Monstrous,” Mirri Maz Duur finished for him. The knight was a
powerful man, yet Dany understood in that moment that the maegi was
stronger, and crueler, and infinitely more dangerous. “Twisted. I
drew him forth myself. He was scaled like a lizard, blind, with the
stub of a tail and small leather wings like the wings of a bat. When I
touched him, the flesh sloughed off the bone, and inside he was full
of graveworms and the stink of corruption. He had been dead for
He says "the women say", not "Mirri says" implying that there were a few women in the tent.