Rhythmic pattern in poetry is used to make it easier on the ear. Different types of poetry have different rhythms and free verse poetry despite not following a particular pattern still uses rhythm in each line to drive the poem forwards.
Some forms of poetry require a particular rhythmic pattern:
Sonnets use a rhythmic pattern called iambic pentameter which means the line has 10 syllables in 5 pairs. In each of these pairs the emphasis is on the second syllable like a heartbeat. You could play the sound of a heartbeat or ask the children to feel their pulses to understand this.
Rap poetry has a syncopated rhythm with the emphasis often falling on an unexpected beat.
The rhythmic qualities of a poem are best discovered when poems are read aloud.