Here is the cast of Mikaelson family.... as kids + (Esther and Mikael as young.)

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ESTHER:

Hayley McCarthy as Esther (Young)

Seen in:

2x03 (Every Mother's Son)

2x05 (Red Door)

2x06 (Wheel Inside the Wheel)

2x20 (City Beneath the Sea)

2x22 (Ashes to Ashes)

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MIKAEL:

Isaiah Stratton as Mikael (Young)

Seen in:

2x06 (Wheel Inside the Wheel) 

However, Isaiah's face was not seen in any scene.

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FREYA:

Elle Graham as Freya (Child)

Seen in:

2x06 (Wheel Inside the wheel)

2x16 (Save My Soul)

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FINN:

Cade Weeks as Finn (Child)

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Voltaire Colin Counsil as Finn (Teenager)

Seen In:

2x03 (Every Mother's Son)

2x06 (Wheel Inside the Wheel)

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ELIJAH:

Perry Cox as Elijah (Child/Preteen)

Seen In:

1x16 (Farewell to Storyville)

2x03 (Every Mother's Son)

2x07 (Chasing the Devil's tail)

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KLAUS:

Grayson Kennedy Hastings as Klaus (Baby)

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Aiden Flowers as Klaus (Child/Preteen)

Seen In: 

1x16 (Farewell to Storyville)

2x03 (Every Mother's Son)

2x07 (Chasing the Devil's tail)

4x05 (I Hear You Knocking)

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KOL & REBEKAH:

Roman Spink as Kol (Child)

Callie McClincy as Rebekah (Child)

Roman/Seen In:

2x03 (Every Mother's Son)

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Callie/Seen In:

1x16 (Farewell to Storyville)

2x03 (Every Mother's Son)

2x17 (Exquisite Corpse)