Goodness Joanna, I am so sorry, that is extreme, is there a reason for them to be so very afraid? If they are not following the guidance from the UK.Gov, which would allow you to meet up inside, socially distanced, unless you are on your own and forming a bubble with them, in which case you can cuddle your grandchildren.
Are they trying to protect you or themselves?
I thought some of our family were rather taking it to extremes, but they will at least come into the garden with us. Perhaps they are worried that the two and a half year old might run up to you, but they could put him/her on reins if that was their worry? 