The statements about not wanting to interrupt their family life, it not being clear why they've run and so on are very odd
They've been to two ports, Liverpool and Harwich, but have apparently not tried to board a ferry.
There are older children, apparently not in her care. Perhaps the police are saying they don't know why they ran because the plan was for close monitoring / child protection plan after the birth rather than immediate removal.
I'm starting to be reminded of Charlotte Bevan, who also went missing with a newborn, and that was mental health based. Sadly they both died. Of course if it was mental health issues, it doesn't explain why they're both running (unless it's some sort of folie a deux and both have MH problems)