get-childitem and folder structures with folder names containing '&'

Sort-Object is doing what is expected, sorts the strings lexicographically, meaning that it compares the strings character-by-character from left to right alphabetically. Their length is only taken into account when one of the strings is a suffix of the other.

What you can do to get the expected output is sort them by their .Length:

$data = @'
MassiveAttack
MassiveAttack/Collected
MassiveAttack&MadProfessor
MassiveAttack&MadProfessor/NoProtection
MassiveAttack/Mezzanine
MassiveAttack/Protection
'@ -split '\r?\n'
$data | Sort-Object { $.Length }

And, if you want to be extra careful, by .Length first then lexicographically:

$data | Sort-Object { $.Length }, { $ }

Which will be useful in cases like this one, where Collected should've come before Mezzanine for example:

$data = @'
MassiveAttack/Protection
MassiveAttack/Mezzanine
MassiveAttack&MadProfessor
MassiveAttack
MassiveAttack/Collected
MassiveAttack&MadProfessor/NoProtection
'@ -split '\r?\n'
$data | Sort-Object { $.Length }
MassiveAttack

MassiveAttack/Mezzanine
MassiveAttack/Collected

etc...
$data | Sort-Object { $.Length }, { $ }
MassiveAttack
MassiveAttack/Collected

MassiveAttack/Mezzanine
etc..

Another trick that should work is to add left padding to all strings so they have the same length, this isn't particularly recommended and is more sort of a hack that works because we know all the strings have length lower than 100... It isn't recommended for big data sets because it has to create new strings which is expensive and consumes more memory.

$data | Sort-Object { $_.PadLeft(100) }

Function Get-ChildDirRecurse {
[CmdletBinding()]
Param([Parameter(ValueFromPipeline=$true)]$Path)
    Process {
        $Path | Get-ChildItem -Directory | ForEach-Object {
            $
            $ | Get-ChildDirRecurse
        }
    }
}
$tree = Get-ChildDirRecurse -Path "/Music" | Select-Object -ExpandProperty FullName
$tree