Now I don't really write for adults or kids - I don't write for kids, I write about them. I think you need to do that, otherwise you end up preaching down.
Preaching a series allows you to go into greater depth in the text, and spending several weeks on one theme allows the teaching to be absorbed more thoroughly.
Preaching is not the performance of an hour. It is the outflow of a life. It takes twenty years to make a sermon because it takes twenty years to make the man. The true sermon is a thing of life. The sermon grows because the man grows.