Game of Thrones has absolutely no qualms about killing kids. This is the show that burned Shireen Baratheon alive, skewered Rickon Stark right in front of his brother, and murdered and mutilated young Ned Umber just a couple weeks ago.
Still, Lyanna Mormont feels about as safe as any child has ever felt on this show, despite her determination to fight in the Battle of Winterfell. She's proven a fearless and exceedingly capable leader for House Mormont and an unswervingly loyal supporter of House Stark, consistently offering just counsel that is wise beyond her years. This has never been a show where merit-based arguments for survival have carried much weight, but if anyone ever deserved to get through a massive battle unscathed, it's her.