Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
"A veces lloramos tantas lágrimas que en ellas podrían nadar ballenas." Llorar mares y que se te queden dentro, ilustración de Paula Bonet http://gnomo.eu/collections/paula-bonet/products/lamina-paula-bonet-ballena