Gap between footer and bricks under family room addition

mtb5458

I have a gap between the footer and the bricks under half of my family room. It was an addition started but never finished when we bought the house. We expanded the addition to go across the whole back of the house. There is no basement, but a crawl space of 1-2 feet under the family room. I am afraid mice or worse could get in through the gap and then into my house. I also think this gap creates movement with freezing and thawing which causes cracks in corners of my house where the addition is attatched . Help.