Since the 1990s, Lahaina's Halloween Party has been the largest celebration Maui has to offer each year. With 30,000+ vacationers and locals coming together on Halloween evening and night.
This year, for 2017, the Keiki (kids) parade beings at 4pm. As someone who was in the parade as one of those keiki for many years, I always remember getting there an hour before had because of the parking and traffic when 30,000 people ascend into the town.
For this free event, spend the money you would have need to spend on some paid event and get a costume. Yes, some people dont dress up, but dont be the odd person out that remains in street clothes. You'll make many more friends with a consume :) Although, even more fun than dressing yourself, is dressing up your kid. Its a great memory you can share with them and safe environment with 80 police officers in attendance.
Once the sun goes down, the adult portion of the night gets started. I wouldnt advise having you kids to be around for this part of the evening, as the streets are often littered with revealing and explicit costumes. Its not the majority, but just be aware that its unavoidable.
Have an amazing time and enjoy Maui's Mardi Gras of the Pacific 2017!