Above all else, don't worry! You may change directions here and there but it's ok. My suggestion would be to keep a timeline forEVER. Whenever anything comes up, whether school related, in church, in life, put it on your timeline. Put their birthdays and other family members bdays on it. Put big news events on it. If you move you could put the dates of the different towns on it. Then if you read Little House put "Little House" on it (the years its about). When you read about Noah find the "best guess" time frame and put that way back there. Don't forget to put world history events on it, too!
As they get older you will fill in more gaps but there will always be these reference points - oh, that happened BEFORE this? Or gasp! Noah was born 56 years before ADAM died?!!! (I think that's right?)
If you go chronologically from the beginning, you will have gaps. If you start with American history, you will have gaps. You just can't cover everything the first time through. It's the layering of learning that helps it stick. Do hands on activities - use playdough or make stick people or make a play. But ALWAYS put it on the timeline.
As for which to do, well, we generally teach kids from their neighborhood out. That doesn't mean you can't teach them about constellations or the earth's place in the universe but you concentrate on the neighborhood. How the fire department works, what they do. How early do bakers get to work? How to make eggs.