I'm wondering why the windows are small...why not 'Mijn vader bestelt ramen voor ons huis.'...?
Because this sentence is in the diminutives skill. :) Since ramen is also a valid translation, it's accepted as well.
Why not 'Mijn pa is ramen voor ons huisje aan het bestellen'?
Except that Mijn pa is My dad, your sentence is a more accurate translation of the English sentence which implies that the ordering is being done at this moment.
So 'Mijn vader is ramen voor ons huisje aan het bestellen' would be accurate, if the father is ordering the windows at the moment of speaking?
Yup! The aan het + infinitive verb combination is one way of expressing that in Dutch.
you could also imagine that "house" will become "huisje" here, couldn't you?