Denelle, I think 8mph with all that gear and kids is pretty good, actually. I'm a strong rider and do something close to 10000 miles a year, and I doubt if i'd be that much faster in town under similar circumstances. For comparison, I did a long tour recently with about 50lbs of luggage on my bike. I probably averaged 12 mph but there were plenty of days when I was down around 10mph.
Look at it this way. If you get used to riding this bike with kids, groceries etc., you're going to get pretty strong. When you get the Trek out for the family rides, you'll be flying.