Stick a two-metre line of tape across the floor, or lay a ribbon down if you would rather not use tape.
Walk it yourself first, arms out, slowly. Then invite your child to follow.
When the straight line is easy, make it interesting: walk it backwards, sideways, or carrying a soft toy on one open palm. Each variation is a new balance problem on the same line.
Walking a narrow line trains dynamic balance and the constant micro-adjustment of the core. Carrying an object adds a second demand, which is how balance moves from something the child concentrates on to something the body handles automatically.