I'm going to write this story in parts, whenever I get the time.

Mika tugged at her mama's skirt impatiently. "Please, tell me the rest of the story!" she said as they walked down the street. She was six then, desperate to learn more about the Roman gods. Mika was an odd mix; her skin was dark, her hair light brown, her eyes a fair green. Her mother came from overseas, and her skin was a deep brown, and curls of black hair tumbled down over her sea blue eyes. Mika's mother was of meduim height, and seemed to often forget things.
"Then the hero followed the string back to the entrance," said Olive, her mother, absentmindedly, fumbling for the keys. They had reached their house, a derelict old building painted green. "Now go to bed, it's past your bedtime." she ordered, locking the door behind her.
"Yes, Olive." said Mika, grudginly. She had never called her mother mama or mum; she just was Olive to her, her parent. Mika had no intention of going to bed. She always stayed up late. As she slowly changed into her too-big pajamas, she heard someone come in, someone with keys of their own. Interested, she crept down and sat on the stairs.

The newcomer was her grandmama, who looked like an older version of Olive. She began to speak, so quietly Mika couldn't hear.
"Olive..." her grandma began gently.
"What?" Olive said in a wary tone.

What do ya think? Constructive criticism please!

So far, I think its good! Well written!
This first bit is short, so its hard to give my opinion...
But I would like to read more! :)