Argthtjtr

When someone is lying down, you say sit up.

When someone is standing in an upright position, you say sit down.

What in the situation when you want to ask a very small kid to sit down to a chair, but the chair is too high for him so he has to climb up the chair to sit down on it?

_{(source: voiceboks.com)}

Sit down sounds like not matching the context. Would you still use sit down?

What I want to find is if for the native English speaker the phrase sit down is stronger than the idea of the logical direction as in sit up (which seems more natural to me as the non-native speaker). So if you really had to choose and you had no other options, what would you select.

(I believe this is not a duplicate of lying down and then sit up/down?)

You would still use sit down no matter how high up they have to climb to get onto the chair. You want them to sit in the chair not stand on it or swing from it or any other things children like to do on a chair other than sit on it.

Get up on that chair and sit down.

or

Sit down on that chair. (no need to let them know they have to climb up to do it)

You also want to be explicit to avoid the inevitable case where the child has climbed up onto the chair and is now spinning around while doing a handstand and when asked why they are not sitting down in the chair they will reply with You told me to get up on the chair, you didn't say I had to sit down on it.

Probably different for adults, where sit up there would be OK to mean sit on the large green chair. See Large Green Chair Minnesota

"Would you please get up on your chair?"

I wouldn't say anything different to a small child (or anyone else) than this. You're trying to force a square peg into a round hole, to use an old expression.

It's like asking whether you would tell the child to eat his breakfast or eat his lunch when you're serving dinner. The only answer here is that in the situation you've given, there is no such choice between “sit up” and “sit down”. People simply do not use either phrasing, at all. This site is about actual English usage, not how it would work in nonexistent, hypothetical situations.

'Sit up here sunshine, (whilst patting the chair with your hand)', or 'sit up on your chair'. But everyone would have their own way of saying this. There is no correct version, though some incorrect ones. I definitely wouldn't say 'sit down' to a toddler unless I wanted them to sit on the floor.

"Sit up" is a valid form of expression, referring to sitting "in a straight, tall position".

e.g. "i was slouching, and the teacher told me to sit up/sit up straight".

As a native speaker, I would always say "sit up" to a toddler, never "sit down" on a chair that is high for him.

If you don't want to use "up" or "down" because it refers to the physical direction of movement, you can try using "on" instead.

Sit on the chair; or

Get on the chair.

Whether the person is taller or shorter than the seat of the chair, what you end up doing is sitting on the chair. You won't sit under the chair. Using "on" can eliminate the referrals of direction implied in "up" or "down".