If the point of this is to instill Humanist values in children, why drag all the labels and complexity into things? Why structure it as teaching the blanket term of "Humanism" to children, instead of just teaching the individual values that Humanism seeks to promote?
Kids learn best by example, not by rhetoric. Instead of trying to teach them about a school of thought, just embrace and exemplify the things that school of thought champions. If kids see adults treating each other as equals, they'll mimic that behavior naturally and easily, without need for instruction or convincing.
They don't even need to have a name for it, or even a proper conception of the behavior as a thing. If a child grows up seeing examples of people treating each other as equals, that simply becomes their baseline understanding of reality. After all, that's how our extant cultural norms get passed down and accepted.
Young children see social divisions all around them as they are growing up and they just accept them as the way of things. No one has to explain to them the concepts of gendered behaviors, or racial and ethnic delineations, or nationalistic divides. They glean it from the behaviors of society at large.
They go to Church with their family every week because Church is just "something you do" - not because they have any particular understanding of the religion itself. They recite the Pledge of Allegiance and sing the National Anthem for the same reason - they don't have any real conception of Nationalism, it's just "something you do". They don't treat men differently than women because they were told to - they do it because the people around them act that way, and it has yet to occur to them that there's any other way of doing things.
So why would you try to instill Humanist values in children in such a backward and inorganic matter? If they even need to have a name for these values, you've already failed to instill them early enough and strongly enough.