Find something low and stable — the bottom stair, a firm cushion — and stand close by.
Let your child jump off with two feet together. Two feet is the skill; one foot is a step down and does something different.
Call the countdown together. Move to a slightly higher surface only when the landing is consistently steady.
A two-footed jump requires both legs to push at once and both to absorb the landing, which is real symmetrical coordination plus the confidence to leave the ground. The countdown lets the child prepare rather than be surprised by their own jump.