The title of the poem is your first clue as to the meaning of this poem. "Intimations of Immortality From Recollections of Early Childhood" is a lengthy meditation on the joys of youth, as being remembered by one who is no longer young.
Wordsworth believed that the newly born human was very close to God, but grew away from that perfection as time went on. Therefore, the memory of the joyous activities and attitudes of youth is related to the recognition of how close the young child is to the pleasures of innocence in Paradise. The quote is encouraging the children to "sport upon the shore" so that adults may
join your throng,
Ye that pipe and ye that play,
Ye that through your hearts to-day
Feel the gladness of the May!
A tabor is a type of snare drum. It is frequently used as an accompaniment for tunes played on pipes, a flute-like instrument, as mentioned in the passage above. Wordsworth is recognizing the response of all nature to the joyous sound of the birds' song and the music being played on the pipe and drum as spring (representing youth) arrives.