[crosshairs]

It looks like she had a failed launch of a snot rocket.

[Miss Behave]

It looks like a stream of tears going down the side of her nose...and it looped around the bottom of her.nose due to her possibly having her head tilted down to the right...maybe she had her in his chest crying right beforehand.

Take a close look...and the solid line of it is noticeable due to light reflection from the sun.

Definately looks like tears to me. It would explain both of their expressions and the way they're hugging.