Police believe the bomb was jammed in the family’s letterbox.
Superintendent Mark Harrison, of Merseyside police, said: “This was a terrible and cowardly attack on a family home, and it is extremely fortunate no one was seriously injured or killed.
“The outcome could have been very different. We are in the early stages of the investigation and working to try and ascertain a motive for the attack and trace the offenders.”
House-to-house inquiries are being carried out and crime scene investigators are at the scene.
Anyone with information should call 0151-777 6564 or Crimestoppers on 0800 555111.
