Any suggestions on how to make our daughters 8th birthday truly memorable in Marseille?
I would say that having your 8th birthday in Barcelona should be memorable in itself. You aren't going to find restaurants that will sing Happy Birthday to her, but you could take her out to have some unusual tapas (my kids actually liked octopus when they ate it in Barcelona). You could get tickets to La Sagrada Familia, and if religous, say a special prayer for her. Take her to the Boqueria (market) on Las Ramblas and let her pick out a treat. (Or get Dunkin Donuts from right next to it) Enjoy the view from the top of the Cathedral. Buy 8 Chupa Chups (lollipops). Barcelona is a great place to go with a kid, and there's lots you can do that will hopefully be memorable.
It's not a stereotypical Rick Steves site, but consider the Tibidabo amusement park (http://www.tibidabo.cat/en/homepage). Although it's best if you and your kid aren't scared of heights, as the park is on top of a cliff and many of the rides take full advantage of that. Spectacular views over Barcelona. Or for something more sedate, Park Guell would probably be interesting.