Hi Rocio,
Thanks for visiting the Disney Parks Moms Panel with your question.
I had the chance to attend
Mickey's Not So Scary Halloween Party with my family, including my 7-year-old daughter, last year. We had such a great time! I do think that it will be very nice for your 4 and 6-year-old children. My family had a great time trick-or-treating around the Magic Kingdom. There are many trick-or-treat stations and trails scattered throughout the Magic Kingdom. We also very much enjoyed the special Halloween parade and fireworks. The parade and fireworks show at the party goes above and beyond the usual Disney parades and fireworks shows, which are amazing to begin with! We also all dressed up as a family, and enjoyed the comments and compliments we heard around the park as a family of fairies, including my daughter as Tinker Bell and my husband and I in adult-sized fairy wings!
I do find the party to be worth it. One suggestion that I have for you, especially since your children are a little younger, is that you do not go to a theme park on the day that you attend the party. This way, you can relax a bit, maybe swim at your resort pool, and then be rested and ready for the party.
The party will likely be a little more crowded on October 29 than it would be earlier in October; however, I would not worry about that too much. That date is a Tuesday, which will likely be less crowded than a weekend or Halloween itself. Also, a limited number of tickets are sold for the party, so it cannot get too crowded.
I hope that these thoughts are helpful for you, Rocio! Have a great time planning your trip, and please do not hesitate to ask if you have any further questions.