May one allow children to play in water on minor fasts?

Question

May one allow children to play in water on minor fasts? Are the rules the same (depending whom you follow) as for an adult?

Answer 1

From the Mishna Brurah 550:5 it seems that children who understand what happened on those days should definitely participate with the tzibbur as much as he can.

Answer 2

The Aruch haShulchan writes (OC 551:31):

אע"ג דאין אבלות לקטן כמ"ש ביו"ד סי' שפ"ד זהו בסתם אבלות אבל באבלות דרבים שהוא זכרון לחורבן בית קדשינו ראוי לחנך גם הקטנים [מג"א סקל"ח] ולאו משום דאבלות זו חמירא מסתם אבלות דאדרבא שיש דברים שמקילינן באבלות ישנה כדאמרינן ביבמות [מ"ג:] ע"ש אלא דבזה יש חינוך ליראת ד' שהקטן כשישאל על מה זה יבינו לו עניין חורבן בית המקדש
Even though [in general] mourning laws do not apply to minors (as outlined in YD 384), this is as regards [personal] mourning; however, for public mourning which is to remember the destruction of our holy Temple, it is proper to educate minors [in this matter]. This is not because the latter form of mourning is more strict, for indeed there are a number of matters in which it is more lenient (as outlined in Yevamot 43b), but rather [doing so] is a way to inculcate fear of God for the minor will inquire what is going on and will come to learn about the destruction of the Temple. (my translation)