Hi Patricia,
My family and I attended
Mickey's Not-So-Scary Halloween Party last year, and we had so much fun! I understand your concern about if it is good for your children given their ages. I think that you are going to need to consider how your family vacations, and what you know about your children, to determine if this is a good event for you.
If your children really like Halloween, or you think that your family would enjoy dressing up in a creative way, I would say absolutely go. Mickey's-Not-So-Scary Halloween Party a time when adults and older children are allowed to dress up in costume in a theme park, which they are usually not allowed to do. We had a ton of fun last year letting our daughter decide on our costumes. She choose that we should all be fairies - so my daughter, myself, and my husband all walked around as fairies for the night!
There are also experiences exclusive to the party. For instance, Mickey's "Boo-to-You" Halloween parade starts with the Headless Horseman riding down the parade route, and Happy HalloWishes fireworks have include amazing fireworks that seem to be all around you. Although entertainment may change for this year, there are sure to be experiences that are only available at the party.
Finally, if your children are not excited by Halloween, the party is a great time to experience attractions with shorter waits. Since there are less people in the Magic Kingdom than usual, the wait times are typically less. Also, you can stay at Magic Kingdom very late. In this way, you could use the party to replace a day of theme park tickets. Spend the day relaxing at your resort and then head to the Magic Kingdom in time for the party.
I hope that these thoughts help you to make your decision, Patricia. Please do not hesitate to ask if you have any further questions!