I agree with Dman, I rhink it's really lovely seeing them out together. They have a very close relationship, and Sarah's definitely the kind of mum who has stayed very "young at heart" and relates well to her daughters' lives.
Also, it's worth noting that they have several friends in common across their 2 generations. For example the Branson family. Also, Vanessa Calthorpe, the stepmother of Isabella C and Cressida Bonas, is one of Sarah's best friends from years back. The Pearson family of Cowdray Park is another example, where Sarah is friends with the parents, and the girls with the children.
So I think it's perfectly natural that they would quite often be seen together at the same kinds of places. The upper class London social scene is a fairly small world after all, where so many people are interconnected through ties of family and friendship or just through hanging about in the same areas. But most of all, I just think it's so nice that they have this kind of close relationship. Rhey always look so happy together, so loving!