From what I've seen, the quality of the Linus isn't a whole lot higher than the Windsor; neither of them inspires much confidence as a daily transportation bike. For recreational rides, it might be good enough. Same goes for the Public bikes.

In that general range, I was quite a bit more impressed with the Priority bikes. If your wife doesn't mind a coaster brake, I'd go with the Priority Classic Plus (me, I hate coaster brakes and would pay the extra for the Gotham edition with front/rear v-brakes).

Honestly, though, I'd look around first for an old Raleigh Sports. Perfect for her needs, bulletproof, available cheap.

That sounds like a terrible idea to me, but I've got no direct experience to back up my statement. Keep in mind that the Burley trailer hitch, at least, is designed to work with a quick-release skewer on the rear axle -- in my (direct) experience, it does not work well with a nutted axle. And like Unca-Sam says, you definitely don't want that weight pushing you downhill on a fixie.

My daughter's only nine months old and still riding in her car seat in our cargo bike, but by all accounts the balance bike is far superior to training wheels. My neighbor's kids started with balance bikes and were riding pedal bikes before they turned 4.

I realize you're probably not interested in buying a cargo bike, but the advantage there is that when the kids get tired, you can load up both them and their bikes and ride home. Just a thought.