I am so sick of fic writers making 10-year-olds talk like babies.
I work with children and have taken numerous classes on adolescent development. Here's some of what I learned because I might actually kill someone if I read another fic where an older child is essentially a four-year-old. (No hate to anyone in particular. Children are confusing.)
(Slight trigger warning for 13 & 14 year-olds. Puberty/sex mentioned)
(Most info is from Chip Wood's Yardsticks: Child and Adolescent Development Ages 4-14)