...may I suggest a song I learned when mine were small? It's the second verse of Summertime by George Gershwin. The words are really loving and so comforting. Once it became familiar my children would relax very quickly, and over the years there were many times when it worked a charm - even with tense teenagers! Go to Youtube to learn the tune!
One of these mornings
You're gonna rise up singing
Spread your wings
And take to the sky
But till that morning
There's nothing to harm you
With Daddy and Mummy standing by...