The big guy that ate Selene tracked them down.
My guess, it's a Keeper of Secrets. And while the soul of the kid might be a nice snack, fighting hordes of Tyranids offers way more ... excitement.
@ Raven Coldheart
The child was on the durchii controlled planet (comic page #80). The Brood King might have decided that the child wasn't a priority. I would love to see Selene's reaction to all of this.
Tyradids are know to make tactical retreats if facing an unknown or powerful enemy. They usually come back with an evolve strategy to deal with the new situation. So retreating like that if sensing a powerful enemy ain't outside the realm of possibilities