When Joss was first writing season 5, Dawn was about twelve years old, and some people say this was what went wrong with the first few episodes. She was much too whiny for her age, and if they had stuck to her being younger it would have been OK. 

Coming from a person who has a younger sister...no, it wouldn't have been. She still would've come off as whiny even at twelve. 

What would've made a much more interesting character would be a little sister, age range 8 to 10, with similar characterization to Lilo in Lilo & Stitch. She was a child character written well, and her weirdness would've fit in the Buffyverse. 

She would've felt drawn to Willow and Tara because of their magic, and Anya because of her own weirdness. Spike, in her eyes, would've been the coolest person ever. Xander probably would've been her least favourite person because of his demon hate. 

Having her younger would also have made Buffy's need to protect her a lot more believable. The younger the character, the more helpless they become. 

The challenge: write a much younger Dawn with different characterization - which somehow leads to a Spuffy relationship.