Are children allowed to go into a kiddie pool with water up to their waist during the nine days? The pool is in the backyard.
Ideally, children should not enter a pool during the nine days at all. Nonetheless, one may be lenient prior to the week of Tisha B’Av, or if they are under the age of Chinuch (2-5 years old). If the water touches less than the majority of their body, one may be lenient regardless.
Still, it should be noted that many are particular to avoid any activities which are unsafe during this period.
ראה בארוכה שאלה 558 ובמ״מ לשם: