Another vote to watch the second show and to watch from FL.
I can't vouch for all the kids leaving early as ours that were 2 and 3 yrs old at the time were still going strong at midnight!! Also when a family has to pay that much for an 4/5/6 tickets they want to get the most out of it plus it's often a one off so you don't want to leave early and miss out on all the shows etc.
I'd also say, wear layers. We went in November a few years ago and as it was sometimes getting a little chilly at night I dressed all of us up in trousers, sweatshirts etc and it was still boiling at midnight!! People still in vest tops and shorts.
As it is your only trip to MK, I'd say to get there as early as possible. We arrived at 5.30pm and it was heaving. I've never been to the MK when it was full to capacity - quite an experience, but a great one.
You'll have a wonderful time, it's so magical.
Jane.