D&D 5E Upping Challenges for 7 Players

[Quartz]

Something else to try is the Mythic Trait from Theros. Basically resurrection with full HP. Or, if you want to be nasty, when the PCs are congratulating themselves, have the other dragon show up. And because the PCs are busy celebrating it attacks fully prepared and with surprise.

 

[Baldurs_Underdark]

The players (teenagers) were laughing around the table until they were snorting and falling out of their chairs.

What kind of laughing are we talking here?

If they were having genuine fun, then that's awesome. The only problem may be that the DM wasn't having fun. Maybe you can accept that some encounters are easier than expected? It seems to be a common misconception on these forums that all encounters (especially BBEGs) must at all cost be super challenging. That's nonsense. Players love a walk in the park every now and then. But if that doesn't sit right with you, discuss with the players how to strike a balance between the real-time duration of encounters and the CR. As these guys level up, this issue will only get more pronounced. A single encounter may take >1 session.

If they were laughing at you, then firstly your problem isn't the CR of the encounter, but you have a lack of respect at the table and you may need to reassess what you're doing here. You cannot play D&D without mutual respect at the table.