for once I am agreeing with Bumbly get your kids tested, if they are immune there is no problem, if they are not then they will need MMR.
There is actually a reason for this, at the time you opted for this there were very reputable clinics offering singles.
There were also some very dodgy clinics set up. I heard of one that was a camper van in a car park.
In both cases, there was a sudden high demand and all the single shots had to be imported. Some were just sent through the post or by fedx so they were fairly useless.
The problem is that unless you blood test every child you do not know their status.
The same is true of MMR, but due to licensing and the way they are transported the government, schools, parents, anyone who cares can be sure 90% are fully immune and the herd immunity will keep the others safe.
If you are immune to one or two or all three of MMR having an extra dose does no harm. You might have a sore arm and a temperature but nothing serious.
Those very rare cases where someone has an adverse reaction are when someone is not immune.
Personally I would just let them have MMR, but as you are worried get blood tests. Yes you may have to pay.
It depends, it's like holiday vax, my GP things it's better that everyone has them so they don't get rabies or yellow fever so does not charge. Many GPs do.
Your dd is also old enough to decide for herself.
At the moment the chances are that your children are immune, but you don't know.
If they are not immune to any of the three then your only option is MMR or no immunity. Does your dd want to have children?
I'm too old to have had MMR, I had mumps as a child. measles vaccine as a baby and rubella jab as a teenager.
But if I was in Swansea, I'd be queuing up for MMR just in case my immunity has waned.
You didn't mention the age of your younger child but dd is an adult, and your other child I'm guessing is a teenager.
Talk to them. This is their risk, and their bodies. As you mentioned school at least one of your children is interacting with a lot of other children who may or may not have been exposed.