"Now they can't seem to accept that we love to treat tto children when we have them/care for them while they're at work. I do honestly wonder if this is all down to the fact of how much they enjoy their time with us. Perhaps by insisting that we deny them things when we have them, that somehow they won't be as close to us. Although we know that our time, love, attention etc is far more important to give them and I believe they would athat too."
My reading of this is that you provide regular childcare for your grandchildren, allowing your son and daughter-in-law to work without the cost of a nursery, childminder etc. Very laudable, and quite a responsibility.
Your son and DIL have asked you not to 'treat' the children. What do you mean by that? What constitutes a 'treat'? Are we talking about taking them to soft play, or, as I suspect, the 'treat' may be along the lines of filling them with sweeties?
You haven't given enough details to know what you're actually talking about, but from the 'woe is me' tone of your post @CanOnlyTry, I suspect you think that you are entitled to override the PARENTS on how your grandchildren are raised. The overblown language - ^"Master Tactition", "under threat", "deny them things" - .