For some reason, while the lock/unlock icon is visible in the layout TOC, it cannot be clicked to lock/unlock elements that are grouped. It's grayed out:
You can lock and unlock by clicking the parent's lock icon, but that locks everything. Sometimes, I want certain sub-elements locked. You do this by selecting and opening the Element panel and checking "Locked" there, but that's a lot of extra steps for something that could be available in the TOC like with parent layers/elements.