You do not say how old the children are. These constant colds, sore throats etc. seem to be a feature of crèche and nursery school life and children do become more resistant to the germs as they approach school age.
Why this has become such a problem is hard to say - I don't remember it as a feature of the 1970s, but the standard of cleaning was higher then in institutions than it is now and I was a school-teacher and like most school-teachers and GPs never caught more than one cold a winter.
But knowing this, does not help you, does it?
If your own health is suffering, you will need to draw a line and say you unfortunately only can have the children when they are well.
I do realise that this places their parents in a hard place, as neither crèche nor kindergarten, or later on school will accept a child who is ill, and parents have only very few days they can take off work in the course of a year.
But we have to be realistic about this. Your health matters to you, and should be equally important to your adult children, and no, I doubt your immune system will suddenly start coping with these germs.
Also, however much we love our grandchildren, they are not our responsibility but their parents'.