$ ruby -v
jruby 9.1.6.0 (2.3.1) 2016-11-09 0150a76 Java HotSpot(TM) 64-Bit Server VM 25.91-b14 on 1.8.0_91-b14 +jit [darwin-x86_64]

$ uname -a
Darwin zero 16.1.0 Darwin Kernel Version 16.1.0: Thu Oct 13 21:26:57 PDT 2016; root:xnu-3789.21.3~60/RELEASE_X86_64 x86_64

class MyClass
end

my_class_instance = MyClass.new

my_class_children = ObjectSpace.each_object(::Class).select do |klass|
  klass <= MyClass
end

my_class_children.each do |obj|
  puts "Child of MyClass before `define_singleton_method`: #{obj.name}"
end

my_class_instance.define_singleton_method(:my_singleton) {}

my_class_children = ObjectSpace.each_object(::Class).select do |klass|
  klass <= MyClass
end

my_class_children.each do |obj|
  puts "Child of MyClass after `define_singleton_method`: #{obj.name}"
end

Child of MyClass before `define_singleton_method`: MyClass
Child of MyClass after `define_singleton_method`: MyClass

Child of MyClass before `define_singleton_method`: MyClass
Child of MyClass after `define_singleton_method`:
Child of MyClass after `define_singleton_method`: MyClass