Hey Howdy Hey Kanna and welcome to planDisney!
I am so excited to hear that you are going to experience
Mickey's Not-So-Scarry Halloween Party in Magic Kingdom. You are absolutely right that tickets to the parks and special events typically are not required for Guests under the age of 3. We did this with your youngest a few years back so I will be happy to give you an expectation based on our experience.
Kanna, just as you mentioned, Guests under age 2 do not need tickets to attend the Halloween party so just like your theme park entrance, all Guests with tickets will tap into the front of the park and you can head on in. No reservations or anything is necessary to get the little ones into the parks so you shouldn't have any issues.
One thing you are not going to want to miss during the party is
Mickey's Boo-To-You Halloween Parade. This parade is full of energy where all of our favorite characters party through the parks dressed in their not-so-scarry costumes. It is super fun and was a huge hit with our girls at the time.
I hope you and your family have a Magical experience on your upcoming vacation. If you have any other questions please let us know, we are always happy to help!
Boo-To-You,
Nick