I suggest beginning with the empathy prayer of the Hasidic master Rebbe Elimelech of Lizhensk (1717–1787).
The underlying idea is that we ask for Divine Guidance to see the good in our friends and not dwell upon their shortcomings. Here is an abbreviated version specially adapted for your preschoolers:
May we see only the good in our friends and not their shortcomings.
May we instruct each other in justness, empathy and genuine love.
Here is a translation of Rebbe Elimelech's original prayer where the Power of Divine Providence is invoked:
Protect and shield us from jealousy between a person and his/her friend.
Let not jealousy enter our hearts and may others not be jealous of us.
On the contrary, place in our hearts the ability to see only the good in our friends, and not their shortcomings!
May we instruct each other in the just and desirable way before You.
And may there be no hate between friends God forbid.
Perhaps you might like to add the musical notation to this prayer by using the tune of he popular Hasidic singer, Avraham Fried (born 1959). His song was published in January 1991 and can be heard on many websites.
For a longer explanation please see my booklet Radiating Love for Healing: Mirror Neurons, Healing Modalities & Jewish Meditative Tradition, Jerusalem, 2012 (Write to me at Natan21@zahav.net.il to receive a copy of the booklet, power point presentation and recording). Here is a brief excerpt adapted from the booklet:
The prayer of Rebbe Elimelech is a practical elaboration of an idea first expressed by Safed Kabbalists in the 16th century and then incorporated as a preface to the daily Morning Service. It is based upon the Biblical commandment (mitzvah) to love one's fellow as oneself (VeAhavta LeReacha Kamocha - Leviticus 19, 18) as interpreted mystically by leading Safed Kabbalists, the Ari, R. Yitzhak Luria (1534-1572), R. Moshe Cordevero ((1522-1570 and R. Hayim Vital (1542-1620). They pictured this mitzvah as a prerequisite to enabling prayer to move up properly towards Heaven because when a community prays together, the souls are united in loving kindness to form one cohesive spiritual entity.
In his book Gate of Kavvanot, 1b, R. Hayyim Vital explains: Before one fixes his prayer in the synagogue, he needs to accept upon himself the mitzvah of "loving your fellow person as yourself", and love each one of Israel as he loves his own soul. This injunction was then incorporated into codified Halakhic writings beginning with R. Avraham Halevi Gombiner (1637-1683), Magen Avraham, Shulhan Arukh, Orach Hayyim, Hilchot Berakhot, 46. He wrote that before the Morning Prayers, one should accept upon oneself the positive mitzvah of loving one's fellow as oneself.
Now back to your original question. I suggest that with this example of a prototype of a friendship prayer, you then ask your young disciples to share their ideas of what to pray for in terms of friendship, understanding and empathy. Maybe have them draw, create or express in their own way what it means to nurture a relationship and delight in interacting in a meaningful way of VeAhavta LeReacha Kamocha.
 R. Elimelech, a student of the Magid of Mezeritch, is a leading figure of the third generation of Hasidic masters.
 R. Shmuel Neta Halevi of Kolin, Mahatsit ha-shekel, comments on the Magen Avraham: If God forbid, there is a divisiveness of hearts, then the souls of Israel are not united above. But when each person accepts the positive mitzvah of loving one's fellow, through their mutual love they are physically in harmony below and bring spiritual unity in the Supernal Universes. This idea is repeated by R. Shlomo Ganzfried (1804-1886) in the Kitzur Shulchan Aruch (12,2) who adds that by unifying the prayer in love and harmony, it becomes accepted before God.
How lovely to celebrate friendship with your students it is certainly an important Jewish value and idea.
In fact friendship was so relevant to the rabbis they once told the following story:when the legendary miracle-worker Honi the Circle-Maker woke from seventy years of sleep, he faced despair because he was shunned by a new generation of scholars who neither recognized nor attended to him. In his suffering, Honi prayed for death to release him from loneliness, prompting an unnamed sage to utter, "Either friendship or death" (Babylonian Talmud Ta'anit 23a)
The benefits of friendship are appreciated by Jewish tradition. Ecclesiastes wrote, "Two are better than one because they have a good reward for their labor. For if they fall, the one will lift up his fellow; but woe to him that is alone when he falls, for he has not another to help him up" (4:9-10).
Friendship is clearly more than a social connection in the Jewish context. Friends offer each other help, loyalty, protection, support, unselfish love, and moral guidance. Judaism defines friendship as one of the primary relationships in life, a tie at times exceeding that which bonds blood relatives.
A blessing is a lovely idea and so though there is not a specific or traditional blessing for friendship I think the idea of blessing one another is a traditional Jewish way of celebrating the joy of others in our lives...and so I would suggest saying/singing the priestly blessing also known as the parents blessing. Here is a version on video with the Hebrew and some lovely music and here is a translation too!
May God bless and protect you
May God make God's face shine on you and be gracious to you
May God turn toward you and place upon you peace and wholeness
Music is a wonderful way to reach children. There is a very sweet song by Doug Cotler called Thank You God. It is on his “It’s So Amazing” CD. If you go to his website, you can listen to it, I believe. I have used this song with young children & at consecration. The verse that is perfect for what you are describing is, “Thank you for my friends & thank you for my toys. Thank you God for watching over all the girls & boys…Baruch ata Adonai, Thank you God”
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