I really enjoyed the Giulietta, and using the Autoeurope voucher system was really quite seamless, actually. In fact, when I first showed up, Europcar upgraded me to an A3, but I decided to stick with the Giulietta since it had been "incepted" and I wanted an Italian car in Italy. Irrational, perhaps...

At any rate, you should be able to fit your two suitcases, pram, and toddler in the Giulietta (or A3, or for that matter 500L). I cannot speak for the others, but the Alfa had a split rear seat, so at the very worst you could fold down one half of the rear seats and create extra luggage space.

My one take on the 500L is that it doesn't seem like much fun to drive. If that matters to you, it might be worth checking out the Europcar options.