Hi Melissa,
Purchase of your Mickey's Not-So-Scary Halloween Party tickets are classed as in advance anytime up until the date of the event itself and as your son is under 3 years of age, he will not require a ticket in order to gain entrance to the party.
Mickey's Not-So-Scary Halloween Party is my family's favourite Special Event and it is filled with lots of special entertainment including:-
Mickey's "Boo-to-You" Halloween Parade
Happy HalloWishes fireworks extravaganza
Many Disney Characters & Villains meeting and greeting Guests in their Halloween costumes
You are also able to go trick-or-treating - collecting treats from candy stations located throughout the Magic Kingdom theme park.
The party is a lot of fun and as the tone of the event is "Not-So-Scary", it is appropriate for children of all ages.
I hope that this information has been of help and wish you a spooktacular trip!