If an internally-geared hub isn't critical, consider an older (mid-90's) steel mountain bike. I'm about your age and have 2 kids (3 and 6) and have used a mid-90's Trek 820 for hauling my kids around. That bike has low enough gears for towing two kids up a hill, an indestructible frame, eyelets if I did want to mount a rear rack (although I had one and had to remove it because it interfered with the hitch for the Weehoo trailer), and is stable with an upright riding position for good visibility. Sadly my 820, which I'd had since high school, got stolen, and I replaced it with a slightly nicer but inexpensive 930 found on Craigslist. A used bike like this is cheap enough that you won't have invested much if your needs change as the kids get older. Here's photos of one 2-kid configuration. I also have a Burley Bee two-kid trailer borrowed from a neighbor.
