Hi Jagruti,
Mickey's Not-So-Scary Halloween Party is such a fun after-hours party at Magic Kingdom! I have attended this party with my family, and we always have a great time. The site that I linked will provide lots information about this event, but let me share some details with you.
Yes, children can dress in costume for this event, and so can adults! There are guidelines regarding the types of costumes that guests can wear on the site that I linked. My family has enjoyed dressing up is costumes that compliment each other. One year, we all dressed up as Disney fairies! It made for a great photo that we used on our holiday cards that year.
There are also attractions open for the event. Not all attractions are open, so do check out the list on the link that I provided so that you are aware of the attractions that you can experience. The lines for these attractions during the event tend to be shorter than during the regular day, so it is a great time to experience popular attractions!
There are a couple of other activities that I really enjoy at Mickey's Not-So-Scary Halloween Party. There are trick-or-treating stations around the park. There is something really fun about being able to trick-or-treat at Magic Kingdom. Also, be sure to take in some of the entertainment offerings. Happy HalloWishes Fireworks is a fantastic show. My favorite entertainment at this party is the Boo-to-You Halloween Parade. It is a fun parade with wonderful music!
If you do decide that you want to go to Mickeys' Not-So-Scary Halloween Party, you can use the link that I provided to purchase tickets. Note that it is offered select nights between August 25 and November 1 this year.
I hope that these thoughts are helpful for you, Jagruti. Please do let us know if you have any further questions!