Impeach him. Twice.

I work in Early Childhood Education, your kids fine. They’re just playing as they explore and learn. Your husbands approach just dismisses their voice and doesn’t achieve anything constructive.

If they’re still acting like this by 5ish I might start to be a little concerned, but honestly I’d expect they’ll get bored of it in a month or two.

The tantrums are also a pretty standard response as they’re learning emotional regulation, alongside trying to be independent while learning more things at any other stage of life. Its chaotic, all you can to is be there to role models and guide them through understanding what they’re feeling and ways they can control their state.

If you really don’t want to play along, the best method is re-direction. I like to ask confusing questions, e.g. “the snow isn’t white” “Oh what colour is the snow? Is it red, I like red because I like red apples. What’s your favourite fruit?” Or you could just point to say a dog walking outside the window etc. just something to get them thinking about a different topic.

It's a natural stage of development. Playing with the idea of reality is part of understanding and accepting the world around them. Let them play in the world of make believe while asking them about what they're experiencing and help them form understanding. *Pointing to blue item "This is orange." "That's orange? What else is orange?" If they demand you agree with them, it should be fine. This is early development. It's way, way more important that they feel safe and loved than what they're thinking and saying. They will grow and if they seem to be too distant from reality, there's plenty more things you can do to help them at that stage.

Vote for the other guy.

But more seriously, have you tried exploration? “What makes you think that?” “why do you think that?” Etc.

Toddlers are notorious for playing the ‘why?’ game, but I used to have fun turning it back on them. It can lead to some quite interesting conversations if you are in the mood, not exhausted etc. Worth trying?

I'm not a parent but a teacher who has encountered similar things with students. This won't work in all the situations you described but you can try giving them affirmation by telling them they are very creative for coming up with cool ideas or stories. This can provide positive attention without conceding the point.

Also for some things like ice cream not being cold, you get explore the idea with them. Cold is, in fact, not a thing, it is just a lack of heat.

I would always agree and amplify.

Toddler: Snow isn't white

Me: really? And all this time I was wrong? Now I see it, it's pink! Unicorn and rainbow pink! And grass is also pink! And water too! Everything is pink!

Just have some good laughs.

Feelings of others is more difficult and you need to be delicate to teach them respect and empathy.

But doing it in a forceful manner often has the opposite effect.

The problem with direct pushing back is that

Not a parent, but personally I would also err on the side of correcting them, as they're still heavily exploring the world. Partially, they're figuring out the world by making statements they believe to be wrong and do actually want you to confirm that they're wrong.

But yeah, if they're throwing a tantrum, they may rather be looking for more input than that. More attention or a playfight of wits even, so to speak.
Asking them "why?" as many others suggested, seems like a good start here.

Personally, I would also try just feeding them tons of information, like if they say that snow isn't white, tell them that it is, because it reflects all the wavelengths of light. Obviously, they won't understand what that actually means, but it gives them something to think about and in the sense of the playfight, they'll be satisfied, too (i.e. defeated and learned something).

My child does this too sometimes, it's sometimes a form of testing their own power over the world and sometimes odd humour .

If it's obviously a deliberately false statement I tend to dismiss them with some form of "Don't be silly" or "Poppycock". Not arguing, but dismissing something as silly can be more effective.

It's fine. I wouldn't just ignore it though. Maybe question why they think these things. 'What makes you sure he's not hungry? Did you ask him?', 'Are you sure she didn't want to sing? I think she has a pretty voice.'

Your toddler is coughing e their desires more than denying facts. If anything, it's an expression of imagination and should be encouraged with guidance away from just telling outright lies.

Is his name Donald, by any chance?

my kid used to deny my own personal opinions, it almost always pissed me off at first. I felt like I was giving so much and I didn't want to give up my opinions too, simple things like me saying that I like something could provoke him to argue that I don't. My spouse helped a lot, we made it into a family joke like he's a little dictator, maybe he picked up on the laughing and learned that it was inappropriate behavior.