No, if the armbands are properly fitted, your kid will not be able to fall forward.
It is important to remember that in order to use the adjustable armbands, your kid's motor skills and muscle tone must be sufficiently developed so that they can keep their balance and their head straight when out of the water.
However, if your kid moves around a lot and is on the smaller side, adjust the inflation of the front section so that they are a little more submerged at chest level.