Simply googling for the phrase 'flex RangeError swapChildrenAt' returns enough reason to follow this advice. Here's some pseudo-code that does work for swapping the depth of two children, A and B:

1. A needs to be at a depth above B. If not, then switch references to A and B in the following points.
2. Remember the depths of A and B
3. Remove A
4. Remove B
5. Add A to the level where B was
6. Add B to the level where A was

As a side note: If you're removing these children from inside either A or B, you'll want to keep a reference to the parent in a local variable since you'll lose it as soon as you remove A and B.