Finally got DS to bed so I've got some time to check my (actually not so) wild claims now!
I can't find any figures for the UK, but the following is quoted all over the interweb:
"Cold sores caused by HSV-1 are very common. The American Social Health Association states that anywhere between 50 and 80 percent of people in the United States have oral herpes. By the time this population reaches the age of 50, up to 90 percent will have HSV-1. Most people get HSV-1 when they are children." From this page, among others
The thing is, that most people dismiss oral herpes as nothing, when actually the first time you get it, it can be quite unpleasant and accompanied by a fever, but as this most likely happened when people were a children they don't remember it. For many people the symptoms are very mild indeed, but for a few it can be severe. There was a very sad case of a newborn who died after catching a cold sore last year. I mention it not to scaremonger, but to try to show that cold sores with babies should be taken seriously.
I took it in good humour on the surface when my friends thought I was mad for not going round with a young baby DS, when one of them had a cold sore, but TBO it did annoy me that they didn't take it seriously at all.
As the OP's baby is 9mo, not a newborn, I suspect the risks are extremely small indeed, and doing what you say would be a good idea - if it's a cold sore at all that is! But then neither me nor you (as far as I know) are Drs.
She says they all have a cold, and one has spots in the mouth. It might not even be cold-sore related, there are other things which could cause these symptoms, and IMO it should be checked. I would call NHS direct, it's what they're there for.
We are so lucky to have free health care in this country, it amazes me how often people procrastinate about contacting their Dr or NHS direct when they obviously have something wrong with them! I'm not suggesting you should go to the Dr with every little cold - far from it - but if my DCs had a symptom I wasn't 100% sure about (e.g. cold with spots in mouth) I'd definitely call NHS direct for advice. What harm can it do?