Get two blank sheets of A4 paper, some sticky tape, pen and ruler.
Place the sheet of paper on the floor flush with the wall and secure with tape.
Stand your daughter in her bare feet on the piece of paper with her heel touching the wall.
Mark with a pencil the longest point of the foot.
Repeat this process with the other foot.
Measure the length from the edge of the paper to the marked point preferably in cms and this will give you the measurement which you can cross check with the communion shoe size chart.
P.S. Not all brands have the same measurements in communion shoes so you will need to cross check your measurements against the brand you are buying.