The biggest mistake to make would be to try to do it all in one trip, especially one which is so limited in time. We did all 4 parks in 3 days but we've "been there, done that, going again", so it's much easier to skip over something. I also agree with skipping Blizzard Beach on this trip.
EMH's are a MUST if you want to see a lot. Don't expect to finish any park by 10 am. That just isn't doable.
Almost every attraction empties into a gift shop. It's very tempting to browse around. Don't. Save your shopping for Downtown Disney or your last day.
Use FastPasses when you can. Have one person get FastPasses for the whole group, while everyone else is waiting in line for an attraction.
Avoid criss-crossing the parks repeatedly. Get a map, and plot your path ahead of time. All WDW parks form loops that can be accessed thru a central hub. Complete each loop before going on to another.
Urge everyone to take bathroom breaks at the same time. Preferrably when one person is waiting in line at a counter service restaurant with everyone's order.
It's easier to get from the MK to EPCOT's Future World via monorail or from EPCOT's World Showcase to MGM via boat than it is to go from AK to any park or from MGM to MK. If you're going to park hop on any days, I suggest doing an MK/Future World Day and an MGM/World Showcase Day. Spend 3/4 of your time at the other parks before going to EPCOT (it opens later and stays open later most of the time).
Have everyone pick one "must-see" attraction in each park and make sure that they get to do it. That way everyone feels happy.
Pay attention to your mother. She may be a healthy 72 but she is still 72. Make sure that she is well-hydrated and if it looks like she is flagging, get her to a place where she can rest in the shade or inside. A commando-style approach to the parks is hard on the very young and the elderly.
Likewise for the 5 YO niece. Tired kids go from zero to meltdown in a matter of seconds. Consider renting a stroller for her. Even if you tell her that you're just going to put you backpacks and packages in it, there's a very good chance that she'll climb into it at some point.
Character breakfasts can be fun and they are great photo opportunities. You can schedule a PS for early (as in before the park opens) or as late as possible in the morning. To book breakfast with Cindy in the castle, you would need to call at 7am, 90 days prior to your date. By 7:02 am, you will be too late. It's a very hard-to-get PS, so don't make any promises to your nieces until you've nailed it down. Figure on at least an1-1½ hours for breakfast with Cindy.
Lastly, know when it's time to give up the itinerary and take a rest. The parks are filled with tired parents who want to get their money's worth and tired children who want to do something else.