I donít think Zara is merching her daughter. I think it was more like they had a size that the child fit into and she wanted to wear a new shirt and be like her mom. The other kids didnít have a shirt and either did Mike.
Mike is wearing a ball cap with the same logos on it though. I don't know about the UK, but as peri pointed out, this sort of thing is ubiquitous in the US. With it being held at Gatcombe, Peter being the Director and the joint organizers being Peter and Mark Phillips there are probably promotional items galore that were passed out to the family. I'm guessing Mia fit in an extra small adult size. She mirrored Zara exactly by wearing jean shorts.
Anne, of course, abstained, and rightly so.
Love this picture!