You need to sit on a stable (non-rotating) chair with a fairly hard surface (sitting on a soft cushion is not suitable). This is due to the fact that when sitting, you need to keep your back vertical (not leaning forward or backward) and your legs should be straight, with the ability to move freely, and not be clamped. In turn, this is aimed at optimizing muscle balance (the load should be distributed evenly across the muscles) and reducing the body's work on the reflex of losing support (unconscious reflex muscle tension aimed at preventing a person from falling forward or backward when he sits - we learn this in early childhood). That is, with this type of sitting, we save energy and strength and gain comfort.
(You can try sitting on a chair and leaning back slightly for some time, you will feel how much discomfort this brings. The same thing, but less noticeably, happens when playing with an incorrect back position, but it takes away strength, and over time it will make itself felt).
Height
You need to sit at such a height that, when you put your hands on the keyboard, the line from the elbow to the wrist is at the same level and parallel to the white keys, the elbow can be slightly higher than the white keys.
Important: when playing, the wrist is at the level of the white keys.
Distance
You need to sit at a comfortable distance, but not close to the instrument, so that the hands can "breathe", making fairly free, often circular, movements, and not too far away so that you do not have to bend your back.
Opposite what to sit
I recommend sitting opposite the notes E and F of the first octave, since this is exactly the middle of the keyboard on most instruments. If in this position it is very uncomfortable to press the pedals, then you can sit opposite the note C of the first octave.
About the position of the legs
Place your legs next to the pedals. The angle of the knee bend is slightly more than 90 degrees. The feet should be in non-slip shoes with a hard sole for stability (this will also be needed later for comfortable pedaling) and rest firmly on the floor. This is necessary for stability, then the upper body is more relaxed.
If your pedals are not initially fixed by the manufacturer, then they (the right and, if available, the left) are located under the keyboard under the first octave and a little behind so that the end of the pedal closest to you roughly coincides with the beginning of the keyboard.
If you find it inconvenient to press the pedals on your piano, since their height is too high, which often happens on acoustic instruments, you need to put a non-slip rubber mat under the pedals. The height from the floor to the pedal should be about a few centimeters.
If the child's feet do not reach the floor, which often happens, then you need to either place the instrument lower, if possible, or put a sturdy box under your feet so that you can rest on it.
Good luck!