I probably wouldn't use the FPs on your MM day unless you are planning to take a long afternoon break--you'll get plenty of rides in without those FPs.
Here's another thought, in particular if you find that the parks aren't so busy (making the FPs less valuable). If your family likes to sleep in, it MIGHT be worth sleeping in a little and then using your FPs on that day. (They say you can claim the FPs up to 11 AM but perhaps not later.) If you're getting to the parks at opening on the other four days (or even three days), it's very likely that you'll get on all the rides eventually with no problems at all. I do think the parks are downright magical first thing in the morning, but if your energy gets low (which happened to me in June even WITH daily afternoon breaks), sleeping in may hold some appeal.
Of course, that said, if you're set on using your MM on Thursday, then if
Disneyland is a high priority, the only day I'd recommend as a sleep-in day would be Tuesday, since Wednesday is not a MM day (and thus you have another chance of doing Fantasyland upon park entry). There's also the extra-long afternoon break option too.
Have a great trip!