Find That Book

My local library had a series of books (possibly a trilogy) that I enjoyed when I was younger in the mid 80’s. I want to track them down to read to my children.

The library never had the first one but the basic plot outline is;
A boy does a weird monster dance to scare his younger sister which accidentally teleports them to a fantasy world. They are separated and the locals treat his sister as a powerful sorceress because of her torch.

I can’t remember the outline for the second book.

In the third book he does the dance but when he’s teleported he’s shrunk to the size of an insect and gains leaf armour made by ants. He eventually meets a wizard from an earlie book that resizes him but he’s a few inches smaller. Him and some adventurers from an earlier book go into a dungeon where they start getting “killed off” with venomous crossbow bolts and nerve gas.

One of the covers is of the boy doing the dance and the back cover summary always discusses the dance/movements in detail.

Any help would be much appreciated.