I'd give H a clear briefing on what's going on. M is learning about the Groclock, so he might get woken up. If he hears crying or shouting, he is not to worry, mummy and daddy are nearby, M is just cross that she's not allowed to get up. He should put on his ear defenders (£10 on eBay) and go back to sleep. Or, if he can't sleep, he can turn on his light and look at books until his own Groclock shows that it's morning.
If he's established sleeping til 7, he might surprise you what he can sleep through. I know they are very different personalities, but Stan is great with instructions. When he hears A crying, the main thing he is concerned with is, is she OK, but if he knows I can hear and am taking care of her, he's fine.