。
There are some boys under the tree.
Literal
Tree [possessive-の] under [at-に] some-counter [possessive-の] boys [subject-が] exist.
何人か ('some, several people') uses 何 ('what') + the counter 人 ('people') + the particle か to express vague quantity — 'an unspecified but small number of people.' This is a productive frame: 何本か ('some [long things]'), 何枚か ('some [flat things]'), etc. The particle の after 何人か turns the whole quantifier into a modifier of 少年.