These Urban/Fitness bikes seem very popular for commuting here in Chicago, as I see quite a few locked up all over the place. My girlfriend has a Specialized Vita (which is in this category) and she commutes, has a rack/pannier and does weekend rides and it seems to be working fine for her.

The 7.3, being part of this category, I think would suit this purpose, though it is not going to be as fast as a road bike, it will be better than a cruiser/upright hybrid. If you want to go a bit more aggressive you could also consider a cylocross bike, which are also popular with commuters, though I have no experience with a child seat on this type of bike.