The smallest number of marbles that can be broken up into bags of $ 18 $ or bags of $ 42 $ must be the least common multiple of both $ 18 $ and $ 42 $. Factoring, $ 18 = 2\cdot 3^2 $ and $ 42 = 2\cdot 3\cdot 7 $. The prime factorization of the least common multiple must include $ 2 $, $ 3^2 $, and $ 7 $, and no other primes. Thus, the answer is $ 2\cdot 3^2 \cdot 7 = \boxed{126} $.