Answer:
Bullion is money, specifically gold or silver that has been freshly mined but not cleaned and made into money.
Clues to the meaning include the fact that the kids need "3 dollars", the speaker doesn't know how they wanted all that "money" at one time, and they had to "raise" it, as in "raising money". When he says, "I hadn't any idea where to turn to raise all that bullion..." he means the 3 dollars.
