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1.For the cloak:
Measure collar size for inner circle.  Center on edge of fabric.
Measure length you want it.  B 
Draw outer circle.
Cut out
2 & 3. For the hood:
Measure from top of shoulder, up over head to top of other shoulder. C
Measure top of head for length of hood, make longer if you want it to come partially down over face.  D
Cut out. 
Fold 2 in half so both D sides are together and sew up one of the C sides to create the hood (3.)
Sew the D side of the hood to the A side of the cloak.
For closures you can use a pin, sew strings on it to tie, sew frogs on it Black Nylon, Plain Frog or anything else you would think appropriate.  For kids I would say the frogs since they would be the easiest to work with and safer then pins.