Jo, from IPS.gov.uk:
Children
Photographs for children aged six and over must meet the full standards set out in this guide.
Photographs of children five years and under must show a clear image that is a true likeness of the child. As young children can be difficult to photograph, children aged five and under do not need to have a neutral expression or to look directly at the camera, but they must face forward.
In addition to the exceptions above, babies under one year old do not need to have their eyes open (however it is preferable for eyes to remain open). All other standards must be met. If the babys head needs to be supported, it is important that your supporting hand cannot be seen.
If you have real difficulty in meeting these conditions, you should visit a photographic studio rather than a photo booth.