If your little ones like Halloween then
MNSSHP is a must do. My son was 4 & had SO much fun! Trick or treating through the park was a blast, there were so many characters out & about and the lines for the rides were nonexistant. Even Mickey was walk on

.
One thing that I discovered is that it gets very, very dark in MK at night. I put a glow bracelet on each of my son's wrists so that I could spot him in a split second.
We're planning an ultra budget trip for this September & we're planning on attending MNSSHP again...in SPITE of the trip being ultra budget

.
Definitely have them bring costumes & dress up. My son wore his blue power ranger costume all night. I swear I was afraid that he was going to dissolve but he didn't complain once. The kids were all adorable & everyone was SO nice & seemed to be having so much fun.
Just familiarize yourself with the MK ahead of time if you can. Since MNSSHP was our first ever visit to the MK we had a difficult time finding our way around & consequently missed some of the party events. I couldn't even FIND the haunted mansion

.
And remember, the party ticket gets you admission to MK at 4PM, though the party doesn't officially start until 7PM. So you can attend a different park during the day & head to the MK from 4pm on.
Have fun!