Probably because they're mentally much older. I mean, you don't stop being your age even if you transmigrate into a kid right?
Ugh it's because they're child? It's obviously kid logic... When i was 6 years old i got into elementary school and felt like i'm much older and different than those kids who were still in kindergarten, when i was 10 years old i felt like a grown up for having 2 numbers in my age, and those who only have 1 number in their age are little kids
Oops i just read the novel, apparently richard is 14, ancia is 10, and blake is 8, richard is a teenager so maybe that's why he refers to ancia as "that child"
Everytime I see a reincarnated manga when they transferred into a kid's body and have a kid rival, WHY DOES THE RIVAL ALWAYS SAY "That child..." YOU'RE THE SAME AGE! WTF DO YOU MEAN "child..." YOU'RE ALSO A CHILD DUMBASS!