njihati

1. (with se) to sway; to swing
   Grane se njišu na vetru. — The branches are swaying in the wind.
   Deca se njišu na ljuljašci u parku. — The kids are swinging on the swing in the park.
2. to rock; to swing
   Mama njiše bebu da bi zaspala. — Mom is rocking the baby to sleep.

"Ljuljati" is a more frequent synonym, especially for swings.