From loli fanservice to dying grandpas...
I don't know if it's a really clever writing (to lull us into a false sense of security and then create a bigger shock) or really poor writing (it was really abrupt). When you consider that the change of mood already happened earlier (assassination attempt on princess) and it was foreshadowed that something bad it's going to happen (goblin's corpse attack, grandpa hearing voices, comments from MC's bro (?) that dungeon is "different" after level 40), I'd say this was properly executed plot twist.
It just feels weird because it's suddenly heavy and MC is "only" 7 years old. But when you think about it - kids around that age were either working or began training in the middle ages... Then you have fanservice and at this point I really don't know if it was really intended by the author or he was asked by his editor to put in some loli panties. We always blame authors for fanservice that feels out of place but I wonder how often it's author's fault and how often they forcedpolitely asked to add more fanservice.