Hi Patricia,
Thank you so much for visiting the Disney Parks Moms Panel with your question.
Mickey's Not-So-Scary Halloween Party is a wonderful event! I have attended several times. Trick-or-treating around Magic Kingdom is of course a lot of fun, but my favorite part is the special entertainment. The Boo-to-You Halloween Parade and Happy HalloWishes Fireworks Show are some of my favorite entertainment of the year at Magic Kingdom.
This event is priced separately, so yes, you would need to purchase a ticket for Mickey's Not-So-Scary Halloween Party even if you were in Magic Kingdom all day that day with a regular theme park ticket. Because of this, guests often choose to save Magic Kingdom theme park admission days for days when there is not a Mickey's Not-So-Scary Halloween Party, thereby allowing you more time in the theme park with your regular theme park admission. My family even likes to make a party day a resort day - relaxing at our resort, swimming at the pool, and then heading to the party that evening.
I hope that these thoughts are helpful for you, Patricia. Have a wonderful time planning your magical vacation, and please do let us know if you have any further questions!