Oral herpes or cold sores is a very common virus that something like 80 percent of the population has been exposed to. Many children have it. If you kissed someone with a cold sore that is oral herpes or herpes simplex 1, you can get it too. You can get it even if they don't have an active cold sore as it could be right before it shows. But oral herpes is not a super bad thing to get. Would not really be worried about this. Maybe make it a rule not to French kiss if you see a sore on their mouth. That's preventative but otherwise, I wouldn't be worried about oral herpes.
It is very uncommon to get syphilis from kissing. It 'could' happen but rarely does. If they have a chancre or syphilis sore, that would be the point of transmission if you had contact with it.
This article may be helpful for you as it discusses STD's through kissing. https://www.std-gov.org/blog/can-get-std-kissing/ Take away is that while it can happen, it is not a big risk. Other than herpes, you will see that all is extremely low odds.