Stevia

A little.

Yes, it can substitute sugar. But it tends to have a certain taste (also depends on the brand you're using).

Substituting about half of the sugar is usually a safe bet. Or using it in things with a strong taste.

Wouldn't use it for baking / in a dough that requires more than a sprinkle of sugar, where sugar is an essential ingredient (but I have used it for pie fillings)

Idk whether it's recommended for small children.

Also:
let's say you'd usually add two spoons of sugar and want to substitute 1/2 of it with stevia.

You can't just substitute one spoon of sugar with one spoon of stevia.

One spoon of real sugar and a quarter spoon of stevia would usually be enough.

this works ime for sauces, toppings, custards, compotes etc...

My DC sometimes drink cordial sweetened with stevia. They find it tastes similar to sugar, but I find that it has quite a nasty aftertaste. I Googled and apparently a small proportion of the population (I can't remember exactly) can taste the nasty aftertaste. So if one of your family makes a fuss and the others don't it's not necessarily just that one is being difficult, it might actually taste different for them.

Stevia is absolutely vile. I've heard erythritol is nicer, but haven't tried it.

I'd rather my kids had sugar than artificial sweeteners.

Stevia does have a taste imo.

But I (and my family) don't taste it if I use stevia to sweeten some things. Especially not if it already has strong flavour (things like chocolate sauce etc)

I'm really pissed off with Heinz because a couple of years ago or so, they changed the formulation of their lower sugar, lower salt ketchup to include stevia. To me it leaves an aftertaste like artificial sweeteners. I would recommend trying the ketchup and seeing if you like it, before embarking on lots of recipes with stevia.

I tried it a couple of years ago when I was dieting.
It is revolting and leaves a horrible after taste in your mouth.
I would rather have no sugar than use it again.