Turn the cut pieces so that the RIGHT sides are
facing each other.
Surge or stitch sides and crotch together.
Measure the arm & cut it the length you need
for the leg. Add a bit for growing room & seams. Cut
it.
The arm/leg will be wide at the top. You
need to make it smaller. Take your childs leg measurement &
divide it by 2 & add a bit for room. Turn the arm/leg inside
out. Mark it.