There is no risk to your 19 month old. The virus is spread via skin on skin contact, so just make sure that your wife is practising proper hygiene after being in contact with her sores etc.

There is a small possibility that the virus can be passed to an unborn child through the birth canal. If she is NOT suffering an active outbreak at time of birth, the chances of her passing on the virus are very small indeed (0.02% if she's had the virus for over a year, 0.05% if it's a recent infection). If she is having an outbreak at time of delivery, then most doctor's would choose to do a C-section to prevent the transfer of the virus. Herpes in newborns can be very dangerous. In Canada/US, an OB/Gyn would put a woman on Valtrex 30 days before delivery to prevent an outbreak. I don't know what the procedures are there, of course.