her behavior is like a reminiscence of their childhoods.

or
her behaviors are like reminiscence of their childhoods.(do i need an "s" on reminiscence)

her ... their ... childhoods

??

about childhood

In the first sentence, "her behavior" is singular, so you would use "a" instead of "are" and "reminiscence" instead of "reminiscences." The correct form would be: "Her behavior is like a reminiscence of their childhoods."

However, if you want to use the plural form "behaviors" in the second sentence, you would need to make a couple of adjustments. Instead of "reminiscence," you would use the plural form "reminiscences." The corrected sentence would be: "Her behaviors are like reminiscences of their childhoods."