The considerate older cousin—who undoubtedly knows what Frozen is— delivered a bit too literally.
Law's Scottish accent (which you can hear if you read closely enough) is what makes this little prank great. "& a went sound no bother"—more people need to say that.
As Law didn't tweet a picture of real Frozen pjs, there's no knowing if his little cousin got what she wanted in the end, like this girl did from her brother.
Families are the best.