For typical riding, you want the seat high enough that their legs are straight when their feet are flat on the ground. For pedal-less training, though, their knees should be slightly bent when their feet are flat. You’ll usually use a wrench to loosen the seat for adjusting, but your bike may come with a quick-release seat latch instead. Lay your hand(s) on their shoulders, back, or neck without gripping tightly. If they need a bit more support, place your hands beneath their armpits.  Your goal is to steady them, not hold them upright or push them forward. Support them instead of holding onto the bike’s handlebars or seat. Instruct them to use both feet to propel themselves forward. They’ll probably be very wobbly at first, so guide their body in a balanced position. Let them manage the handlebars so they can get used to controlling them while moving.  Catch them and guide them down whenever they start to fall, instead of holding them upright. Otherwise, you are simply replacing what training wheels do. Once they get the hang of coasting, tell them to use their feet to stop when they begin to slow down. Their instinct will probably be to look down at their handlebars or front wheel, and possibly even the pedals later on. Practice having them look out ahead as they coast forward. If you have a second person available, have them stand several feet/meters in front of the bike and back up as the kid coasts forward. Tell the child to watch that person. Once the child is able to coast in a balanced position for as far as their foot-on-ground power will propel them, they’re ready for pedaling. Put the pedals back on as per the bike’s instructions, and raise the seat so that they are flat-footed and straight-legged when straddling it.

How would you rephrase that briefly using Chinese?
将座垫调低一点，让孩子可以踩在地上。 扶着孩子，不是扶着车，也不要太用力。 让他们在你温柔的辅助下慢慢滑行。 教他们往前看，而不是往下看。 将脚踏板和座垫归回原位。