Learn Russian: Иди сюда, детка.

The colloquial truncated vocative is mostly limited to given names (Саш, Маш) and close family terms (мам, пап, дядь). General terms of endearment like детка (baby, honey) or зайка (bunny) don't naturally shorten this way and just use the standard nominative form when addressing someone.

Why use the imperfective verb иди rather than a perfective verb to tell someone to come over?

Imperatives of imperfective motion verbs like идти are standard for immediate, direct commands to start an action right now. Using a perfective verb like подойди (come up to me) would sound more like a specific request to approach or step closer, rather than a natural, welcoming "come here."

Just to refresh, why is "here" translated as сюда and not здесь?

Russian makes a strict distinction between location and direction. Здесь is used when someone or something is already located "here" (answering the question где?). Сюда means "to here" and is used with verbs of motion to show the destination (answering the question куда?).