How lovely, our first nightmare with hiding in combat!

[Ovinomancer]

Two different abilities. The Halfling can Hide behind larger creatures, but they must use their Action to do so (so they'd Hide in the first round, then pop out from behind the Fighter in the second round to attack with Advantage.)

Partly true. Lightfoot Halflings can hide behind larger friendly creatures, but there is no requirement that they must use their Action for this. Normally, it does require an Action to Hide, but if the Halfling has Cunning Action or a similar ability, they can use a bonus action to Hide behind a larger friendly creature.

Whereas the Rogue and Goblin can both Hide as a bonus action, which allows them to Hide first with their bonus, then pop out and use their Action to attack with Advantage. However, they need to be in Heavily Obscuring terrain to do so (which a larger creature isn't-- it is considered Lightly Obscuring terrain.) This hiding and attacking can be done every round for the Rogue and Goblin.

Partly true. Rogues of second level and higher and goblins can hide as a bonus action, yes. The bit about Heavily Obscured, however, is not in the rules, but is a valid way to rule "not clearly seen," which is in the rules.

 

[cooperjer]

But then how will my players know I'm upset with them if I can't have a Tarrasque hiding behind a bush attack them?

They'll know when you tell them that all ranged attacks against the prone tarrasque are at disadvantage.

 

[Ancalagon]

But then how will my players know I'm upset with them if I can't have a Tarrasque hiding behind a bush attack them?

... now I want to run a micro tarasque - just as powerful as the real one, but small like a goblin

"What's that weird critter? It's kinda cute in a OMG IT JUST BIT THE DWARF IN HALF!"

 

[Li Shenron]

Yes, this sounded like it went pretty fine, what was the nightmare part?

It definitely was there behind the DM screen I was super-tense trying to remember all the rules nuances, not to miss rolls, adjudicating characters movement around the room and still be able to keep things easy enough for the kids i.e. not getting stuck and keep as much as possible transparent to them.

I did of course goof things here and there. It took me a few rounds before settling to a consistent adjudication of the whole goblins hiding. The good thing was that the players probably didn't even notice.

 

[Oofta]

... now I want to run a micro tarasque - just as powerful as the real one, but small like a goblin

"What's that weird critter? It's kinda cute in a OMG IT JUST BIT THE DWARF IN HALF!"

Hmmm ... maybe a super powerful alter self? But give the PCs a hint they're still in danger?

Something like:

 

[Oofta]

It definitely was there behind the DM screen I was super-tense trying to remember all the rules nuances, not to miss rolls, adjudicating characters movement around the room and still be able to keep things easy enough for the kids i.e. not getting stuck and keep as much as possible transparent to them.

I did of course goof things here and there. It took me a few rounds before settling to a consistent adjudication of the whole goblins hiding. The good thing was that the players probably didn't even notice.

Don't worry about it. I've been DMing for years and I still feel like I don't know what I'm doing all the time. I ask my wife about it later (and trust me, she has no problem telling me when I screw up) and nobody notices. In most cases it's more important to keep the flow of the game going even if you're just kind of winging it than to flip through rulebooks during the game looking up rules.