R.T.
Try Marcello's at 645 W North Ave. They are super family friendly, VERY reasonably priced and they have a breakfast buffet on Saturday and Sunday.
My second reccomendation is Max & Benny's at 332 E Illinois St in Streeterville. While they don't have a breakfast buffet they have wonderful breafasts with reasonable prices. Portions are huge, so someone can almost always share!
Both restaurants give crayons and coloring sheets when they seat you. :)