1 n. — Sports
a level of sport for children up to 10, usually 9-10 in hockey.
Type: 3. Semantic Change — This meaning is an extension of the meaning of atom as 'any very small object (without the implication that it is a particle of something else); a (relatively) tiny person or thing' (OED-3, s.v. "atom" (9a)). The age range varies from sport to sport and over time, see, e.g. the quotations from 1968 (7-10) and 1989 (under 8), or the 1998 quotation in meaning 2 (9).
See also COD-2, s.v. "atom" (2a), which is marked "Cdn", ITP Nelson, s.v. "atom" (4), which is marked "Hockey".