You are obviously feeling quite defensive now, Kali2, which is a bit of a shame because I think most people aren't so much picking sides as putting another viewpoint. However, if you are feeling disappointed that you will not see your son and grandchildren, it is probably colouring your feelings too.
I know it has been such a long time and I am lucky that my children don't feel the same way as your DIL but many people want more time to see what happens before they start mixing. In a way, it is understandable because the last relaxing in rules started the process for other lockdowns. That doesn't mean your DIL has health anxiety, she might just be wanting to be a little more cautious. Your son can help his children through the feelings which will arise from his wife's caution and, in doing so, he can acknowledge that caution is still necessary whilst reassuring them that it will be ok in the long run. Your grandchildren are getting a valuable lesson in consideration for others too. If they show signs of anxiety, that can be dealt with when it happens.
I think I might be inclined to reassure my DIL that we can go at her pace and are really looking forward to seeing her and the family when she feels able to. You can show empathy and make her feel reassured or you can make waves which will backfire in the long run and possibly cause irreparable damage. You sound like you are leaning towards patience so keep reminding yourself of the long term goal which must be a good relationship with all the family.