Learn English – Need synonym for beginning something eagerly

I am trying to find a one-word synonym for the phrase, "They threw themselves into [an activity]", or "they launched ambitiously into..," or "They zealously began to…".
The actual phrase that I am trying to complete is,

"The boys immediately ______ to construct an arsenal of snowballs."

The word "began" would work, but it's a weak word. I want something more passionate; more earnest. A word that suggests that the boys dove wholeheartedly into this effort. Any suggestions? (Or perhaps a way to reword the sentence altogether?)

"Endeavored", maybe, but this is for a kid's book, and I'm not sure how accessible that word would be.

UPDATE: Wow, I never expected such a flurry of fantastic suggestions. You all are a veritable snowstorm of thesaurania. I find myself now in the position of overthinking this phrase even more than I was before — writer's freeze, as it were. But that's the life of an author, eh? Thank you all for your input.