Perhaps the Child Actor being "undod" is not saved correctly?
Result:
Crash: Assertion failed: Index+Count<=Array->Num() [Link Removed] [Line: 121]
UE4Editor_UnrealEd!FTransaction::FObjectRecord::SerializeContents() editortransaction.cpp:122 UE4Editor_UnrealEd!FTransaction::FObjectRecord::Save() editortransaction.cpp:165 UE4Editor_UnrealEd!FTransaction::Apply() editortransaction.cpp:440 UE4Editor_UnrealEd!UTransBuffer::Redo() editortransaction.cpp:892 UE4Editor_UnrealEd!UEditorEngine::Exec_Transaction() editorserver.cpp:5081 UE4Editor_UnrealEd!UEditorEngine::Exec() editorserver.cpp:5527 UE4Editor_UnrealEd!UUnrealEdEngine::Exec() unrealedsrv.cpp:624 UE4Editor_MainFrame!FMainFrameActionCallbacks::ExecuteExecCommand() mainframeactions.cpp:930 UE4Editor_MainFrame!TBaseStaticDelegateInstance<void __cdecl() delegateinstancesimpl.h:1017 UE4Editor_Slate!FUICommandList::ConditionalProcessCommandBindings() uicommandlist.cpp:211 UE4Editor_Slate!FUICommandList::ProcessCommandBindings() uicommandlist.cpp:155 UE4Editor_MainFrame!FMainFrameActionCallbacks::OnUnhandledKeyDownEvent() mainframeactions.cpp:176 UE4Editor_MainFrame!TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<FReply() tuple.h:115 UE4Editor_MainFrame!TBaseStaticDelegateInstance<FReply __cdecl() delegateinstancesimpl.h:948 UE4Editor_Slate!TBaseDelegate<FReply,FKeyEvent const & __ptr64>::Execute() delegatesignatureimpl.inl:521 UE4Editor_Slate!FSlateApplication::ProcessKeyDownEvent() slateapplication.cpp:4528 UE4Editor_Slate!FSlateApplication::OnKeyDown() slateapplication.cpp:4430 UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() windowsapplication.cpp:1419 UE4Editor_Core!FWindowsApplication::DeferMessage() windowsapplication.cpp:1930 UE4Editor_Core!FWindowsApplication::ProcessMessage() windowsapplication.cpp:747 UE4Editor_Core!FWindowsApplication::AppWndProc() windowsapplication.cpp:669 user32!<Unknown> user32!<Unknown> UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() windowsplatformmisc.cpp:905 UE4Editor!FEngineLoop::Tick() launchengineloop.cpp:2788 UE4Editor!GuardedMain() launch.cpp:156 UE4Editor!GuardedMainWrapper() launchwindows.cpp:126 UE4Editor!WinMain() launchwindows.cpp:202 UE4Editor!__scrt_common_main_seh() exe_common.inl:264 kernel32!<Unknown> ntdll!<Unknown>
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34380 in the post.