As a retired microbiologist, I know a bit about “germs”. While it is true that there are a lot in the mouth, most don’t live elsewhere in or on the body. Kissing a baby on top of the head or on another part of the body apart from face is very unlikely to transfer Herpes, though advice to avoid kissing young babies is valid.
Callistemon - Herpes can survive for a short time (a few hours) on hard, inanimate objects, theoretically, a toddler with a cold sore could infect a toy and transfer to 6 month old who puts everything in their mouth. Fortunately by that age they are less vulnerable to severe infection. PoppyBlue and Hithere - toddlers under 2 can’t understand logical explanations of how they should avoid making their baby brother/sister sick. Some siblings are less than 2 years apart and it’s not practical to keep isolated from each other.
Since science has been mentioned on this thread, it was incorrectly stated upthread that a baby’s immune system doesn’t develop till after weaning. The immune system starts to develop immediately after birth though isn’t fully developed for about 6 months. It’s sufficiently developed for current immunisations to start at 2 months. If the baby couldn’t respond and produce own antibodies, there would be no point in giving vaccines so young.