It’s an attempt at studied neutrality (better than her continuing to smile) and I think it’s more likely she’s being watchful in case anyone tries anything. I know there’s security etc etc but if i were sat there with my kids, I’d be keeping an eye out too.
Whatever you think of the monarchy, the kids being present etc etc, W&C do seem to think hard about what they do and don’t take the children to. They will have explained to them about protesters and be comfortable the children understand.
And whatever you think of Republic, they have the right to peacefully protest which they did. They will have made a lot of noise when everyone else went past too, but Catherine and the children by far will generate the most clicks. And comment.