Do the Fantasmic Dinner Package.
I've been to Mama Melrose, but I hear that Hollywood and Vine is a little more kid -friendly.
If you do the FDP:
1. You can show up 20 minutes beforehand and still get a seat. If you show up earlier, the better your set will be. There is no reason to show up any earlier than 40 minutes before the show starts.
2. The seats are to the side, but are still quite good.
3. One unadvertised advantage to the FDP seats is that you get out of the park quicker after the show is over (the exit is closer to the parl exit and less people use it). Also, I saw strollers parked in single file along the path to the FDP entrance - which I assume would make it easier to find your stroller than the regular Fantasmic entrance. Thus, no waiting in line for a tram to get back to your car, or no waiting for the next bus to come to bring you back to your resort - this will be a big advantage for you tired 3 year old.