Brats (Deadly #2) by Morris Gleitzman

In no time at all we were covered with scampering, crawling, hopping insects and spiders.

They were in my hair and eyes and earholes. I gasped and sucked in air. And with the air I drew in a huge spider.

It squirmed and wriggled inside my mouth like a live, damp feather. I  spat it out with a scream ...