Grannylancs, hopefully some of the advice on here will, as you say, have given you food for thought.
Looking at your last comment you still haven’t quite grasped what people here are advising you though.
“I didn’t go out of my way to give the baby a cold.......I just wanted to see my GS”
In your son and DIL’s shoes this one section would (for me) sum up the issue. You assume this is perceived as a thing you’ve done on purpose - it’s probably not what they think, but it was very careless and selfish. Which is then backed up by the fact that you “just wanted”, your wants overrode theirs and more importantly the wee ones needs.
My advice would be to have a think and and see if you can appreciate their perspective? If pre-pandemic you were cavalier in your attitude to health/safety of their child, then that may be the basis of mistrust during the pandemic.
You obviously feel left out and that’s a horrible feeling for anyone, maybe try and put the emotion aside and look at the issues. If you can see where they’re coming from you might be in a better position to find some perspective.
If you do decide to apologise, make sure you actually know what you’re sorry for and don’t try and justify your actions - you’re right you can’t change what happened then, but you can control what you say and do moving forward. Saying I’m sorry and then following it up with a but or a justification isn’t a real apology - acknowledging where you went wrong, and how you’ll ensure it won’t happen again might give them some peace of mind. IMO you need to acknowledge where you went wrong.
It’s been a tough year for everyone, so I hope you and your mum and husband have a nice festive period and when emotions cool down a little, you’re able to get things back on track with your DS and his family.