Balloons

38. Inflated balloons. A balloon filled with air is normally tied shut. One may not inflate a balloon on Shabbos out of concern he might make a lasting knot; consequently, they are also muktzah on Shabbos (קצוה''ש סי' ק''י בבדה''ש סקט''ז ). However, children who play with balloons by filling them with air and emptying them without making a knot may play with them. They should be cautioned not to make knots (שו''ת באר משה ח''ב סי' כ').

39. Soap bubbles. Strictly speaking, there is no issur to make air bubbles with soap water on Shabbos, as they do not last. However, one should not tell a child to make such bubbles, as some are machmir (הגרשז''א, שש''כ פט''ז אות ל''א ).