If I were you, I'd watch Fantasmic, see the lights after, then grab a taxi right outside the MGM gate. It would be a really cheap fare for them to take you to the Beach Club, which is really pretty much right across the street and past the Boardwalk entrance. Then walk through the Beach Club lobby and head left through the International gateway. No walking, no waiting and minimal cost.
Good luck. I hope you can see it all. My Mom will be there next week and this is what I suggested to her too - she has the same plan as you.
Oh - and also, something that would help is sitting near the top center on an aisle at Fantasmic. Then, at the end when Mickey says, "Some imagination, huh?" - grab the kids and beeline for the exit to get a 5-second jump on the crowd. It will make all the difference in the world. Then you can head over to the lights and be first in.