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.

Solution 1:

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

[2.5] (Of a person) make a sudden, impulsive rush to do something:
Gordon jumped to my defence

Reference:
http://www.oxforddictionaries.com/definition/english/jump

Solution 2:

Given that changing the structure of the sentence is an option, how about something like:

With a flurry (or in a flurry of excitement), the boys began to construct an arsenal of snowballs.

Flurry means:

a sudden short period of commotion or excitement.

It is especially nice in your context, which involves snowballs, since it puns on a second meaning of flurry:

a small swirling mass of something, especially snow..., moved by sudden gusts of wind.

Given this playful pun, it seems well-suited to a children's story.

Solution 3:

"the boys immediately hurried to construct an arsenal of snowballs"

Hurry — ODO

verb 1.3 Do or finish (something) quickly or too quickly
"formalities were hurried over"
"Another factor pushed them to hurry the project: the need to get their ducks in a row before they ran out of time."

Solution 4:

This isn't exactly what you asked for. I would probably dump the word "immediately" and say something like:

The boys began eagerly to construct an arsenal of snowballs.

The word "eagerly" contains the idea of immediacy, as do some of the other words that have been suggested, and so the word "immediately" becomes redundant and weakens the sentence.