shloka for peace and unity

Om saha navavatu, saha nau bhunaktu
Saha veeryam karvaavahai
Tejasvi naa vadheetha mastu
maa vid vishaa va hai
AUM shaantih, shaantih, shaantihi

Meaning:
Let us together (-saha) be protected (-na vavatu) 

and let us together be nourished (-bhunaktu) by God's blessings. 

Let us join the our mental forces together for strength (-veeryam) to benefit humanity (-karvaa vahai). 

Let our efforts at learning be filled with light (-tejasvi) and joy, and be driven by the force of purpose
(-vadhita mastu).
 Let us never (-maa) be poisoned (-vishaa) with hatred for anyone. 

Let there be peace and serenity (-shaantih) everywhere. 

Om Saha navavatu.... is a very melodious and peaceful shloka. It is often sung at the end of prayer gatherings. It is repeated several times while people offer arti or flowers after a prayer meeting. When many voices join together to sing this shloka the effect is beautiful!
This is a good shloka to teach to kids. It is important to explain the meaning to them. Chanting this as a group seems to fill the room with positive vibes and energy. My bal vihar group used to chant this to the accompaniment of an electronic tanpura and the beauty of unified children's voices is priceless!  "A thing of beauty" and "a joy forever" (John Keats)!