First of all, why do you need to find a child from a parent if the child has a UNIQUE id? The main purpose of the identifier is to provide the flexibility of searching for an element using a unique selector.
If the child is actually nested in another, find it using xpath. I often use this.
@FindBy(how = How.XPATH, using = "//something/something") private WebElement TestElement;
Or with id
@FindBy(how = How.ID, using = "abc") private WebElement TestElement;
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