Being Canadian, Halloween has always been part of my life. I love seeing the children dressed up and so excited.
I remember my trick or treating days fondly. Running all over the neighborhood trying to find the family who were giving out those elusive candy apples. Oh, the rumors! I don't think we ever found one.
In those days, there were no fears that nasty people would tamper with Halloween treats.
18 months is kind of young, but there are some good suggestions here. At about that age my DGS was dressed up as a cat. His Mum used face painting to make whiskers etc. so some face paint might work
Also, a Halloween themed colouring book and big fat crayons. There are colouring books for very young children.
A Halloween sticker book might go down well.
Stuffed animals. A black cat is a Halloween symbol. as are bats. And of course witches. There are good witches. I have a stuffed kitchen witch from Salem Massachusetts hanging in my kitchen for good luck. Complete with wooden spoon for a broom
You could get a pumpkin themed little basket for trick or treating and pack things in it.