“On Children” by Kahlil Gibran comes from a text titled The Prophet implying that the poem is part of a larger body of spiritual poems that seek to impart wisdom and learning on those who read. The intended audience would, therefore, be anyone who is trying to learn or who might need imparted wisdom. The audience implied by the poem is parents, as the speaker in the poem talks about raising children and the future of life.

The message of the poem is straightforward. The speaker imparts wisdom about how to live Life, and how to raise children, talking about how the listener should view and act in the present. He says,

You may strive to be like them, but seek not to make them like you.

For life goes not backward nor tarries with yesterday.

You are the bows from which your children as living arrows are sent forth.

The speaker in the poem is clear that the listener should live for the future, and that means preparing your children for Life (a personified force—something that all people belong to) and their world. The message is clear that children do not belong to parents, and that they shouldn’t be forced to take on the ideas and peculiarities of their parents. The speaker explains,

You may give them your love but not your thoughts,

For they have their own thoughts.

You may house their bodies but not their souls,

For their souls dwell in the house of tomorrow, which you cannot visit, not even in your dreams.

You may strive to be like them, but seek not to make them like you.

The speaker imparts to the listener that they should be letting their children make their own choices and become their people because they will have to prepare for a world their parents never imagined. This wisdom comes down to the listener and flies in the face of conventional wisdom—generally, we think parents should raise their children to follow their lead and believe their beliefs.

The tone of the poem could be described in two ways. It might be seen as condescending, depending on if you think the speaker considers those who are listening as dense. The advice given is mainly common sense—at least the points about the children facing a new world and having to create their way. The poem, though, doesn’t give off the vibe of being sarcastic or antagonistic. Instead, I think a better word to describe the tone is benevolent. The speaker is trying to impart crucial wisdom to the listener, and it shows in how they use metaphors and figurative language to get the message across.

The message of "On Children" from The Prophet by Kahlil Gibran focuses on the idea that children are their own people. The implications of this idea lead the speaker to advise against indoctrinating children and to advise against parental attempts at shaping the child to be like the parent. 

"You may give them your love but not your thoughts, 

For they have their own thoughts."

The message is one that advocates being supportive of children through love, shelter and strength. Also, the poem implies (and promotes) the idea that individuality is part of the way that (1) children come to exist fully as the most valuable version of themselves and (2) children exist as part of a larger (mystically imbued) world or, as the poem puts it, "Life."

"Your children are not your children.

They are the sons and daughter's of Life's longing for itself."

This metaphysical vision of the individual's relationship to life and to the world is central to the larger text of The Prophet and is fully part of the book's nature as a wisdom text. While there may be some debate as to whether or not a relatively contemporary work like Gibran's was intended to be a wisdom text, it does bear many of the characteristics of texts of this kind. 

Offering an extensive articulation of a world-view that encompasses philosophical perspectives on the nature of being and also metaphysically inflected yet actionable approaches to social institutions like marriage and parenting, The Prophet takes on the character very much akin to Thus Spake Zarathustra and The Republic as well as The Bhagavad Gita and The Torah.

As in these renowned works of philosophy and religion, The Prophet "focuses on human relationships—with others, with nature, and with God" (eNotes).

The intended audience for such works is hard to define in narrow terms as these texts seem to appeal to philosophical seekers of all sorts. They appeal to a very general audience as well. We might narrow this poem's audience down somewhat by identifying its advice for parents, yet the poem's overall message about an individual's relationship to those around him or her remains quite broad.   

The tone of this poem, like others in the book, is prophetic, exhortative and highly rhetorical. The poem reads, largely, like a speech.