There is a need to worry.
Instead of the usual phrases a mother uses to comfort her child, (There is no need to worry). Kay begins the final stanza with the opposite. This is unexpected and sounds awkward, mimicking the speaker's anxiety. She says she "cannot lie" to her daughter. Perhaps this is because the "terror" is too great, but it might also be because she wants their relationship to be a truthful one. This contrasts to the men in the poem who are covering up their harsh actions with disingenuous "smiles".
a slow light tails the fast car.
The paranoia continues in the line and the sense of things being the opposite of what they seem, or what they should be. Here the opposites "slow" and "fast" imply that however speedily she tries to get away, she will always be followed; "they" will be in the background gradually getting closer.
In response to this, Leila "tugs at her coat", wanting reassurance. Her mother "whispers/ her cradle song" which is a song from childhood imbued with security. As a result, she "holds on". Holding on could be taken to mean both literally holding on to her mother but also holding on meaning to endure difficult circumstances. The half rhyme between "song" and "on" emphasises the connection between the mother's gift of hope and the child's survival. Kay ends the poem by suggesting that whatever the terrible circumstances they are in, Leila and her mother will be there for each other.