One of the most commonly asked about costume is a Nun's Habit. Unfortunately I have not seen a pattern for one since sometime before 2005. Necessity being the mother of invention, I went on a search. Although I didn't find a child's nun costume, I did find a "convertible" options.
Your first step is to look at and analyze Nun Costumes
Next look at the various pattern options that are available. Changing the color and shape of the edge of a collar can instantly convert pattern intended for other costumes.
The patterns listed here were all found in the costume section of sewingpatterns.com
- Butterick 3820 Unisex robe and collar @ Sewingpatterns.com
- Butterick 4542 Children's/Boy's/Girl's Robe and Collar @ Sewingpatterns.com
- Simplicity 4795 Nativity costumes available in misses, men's or teen boy's sizes @ Sewingpatterns.com
- Simplicity 4213 @ Sewingpatterns.com contains a head piece and robe that could be a nun's habit.