Children will be children vs Children are children

Solution 1:

Will here has a repetitive sense rather than a futurive sense. You have probably encountered this will more often in the past tense:

When I was in high school we would always go down to the Southside Grill for cheeseburgers and shakes after school.

You can also recognize this will in present-tense main clauses qualified by whenever clauses, or if or when clauses with a whenever sense:

Whenever she's annoyed she'll screw up her mouth like this and talk very flat.
When you plug this in the wrong socket you'll see this little blue light flashing on the main panel.

You could think of Children will be children as a sort of implicit whenever statement:

[Whenever they are whatever,] children will be children.

A related use employs a heavily emphasized will to imply that somebody's annoying behavior is not merely habitual but consciously and perversely so—they insist on behaving this way:

Frank will put commas after his conjunctions. I'm getting tired of taking them out.

This can also be used with single events; in that case it implies that the behavior is characteristic:

If you will tell the Dean he's an incompetent scholar you've got to expect some repercussions.

Solution 2:

Sometimes will is used to describe "timeless truths," or things that are always true, in 'quaint proverbs' or other maxims:

Boys will be boys.

When the cat's away, the mouse will play.

A drowning man will clutch at straw.

Faith will move mountains.

Love will conquer all.

If you build it, he will come.

If anything can go wrong, it will.

Children will be children.

In these statements, will does not express future time but habitual aspect.

You can insert the adverb always after "will" in any of these statements and the meaning will be the same.