I lost the habit for awhile, in part because I didn't know what to say. The prayers offered on behalf of a child in respiratory distress would be significantly different from prayers offered as police respond to a homicide.
At some point when Peter was an infant, I read through the Liturgy of the Hours in one of my prayer books. The opening prayer struck me as a solution to my loss for words. In our family, we now say the following prayer - a slight variation of Psalm 70:1 - whenever we hear a siren:
God, come to their assistance.
Lord, make haste to help them.