If I notice a child had a crusted sore on their face, I contact one of their parents without hesitation (it's happened just once).
My first was/would be impetigo, which is highly contagious and for which children should be excluded. If for no other reason OP, you should ask that a parent collect the child so that impetigo can be ruled out.
The parent I contacted wasn't pleased, and it turned out not to be impetigo - but I insisted that impetigo should be ruled out before the child returned to the setting.
I hadn't considered cold sores as I've never cared for a child with one, and as people say it's not an excludable condition. BUT, having read this thread and gone onto WebMD to check it out, I'm going to add it to the list of conditions I will exclude for - HPA guidance notwithstanding.
The virus that causes cold sores can't be cured - once you've got it, you've got it.
It isn't just a matter of children kissing and touching each other (and you), which of course you can't prevent; the virus can be spread through shared touched items such as eating utensils - and toys.
I wouldn't want other children in my setting put at risk of contracting the virus, and I certainly wouldn't want to contract it myself.
Any parent who was 'fuming' at being asked to collect their child because of a cold sore or any other suspect sore would just have to fume