Why would Draco facepalm at his wife's brilliance?

The nearby children are watching as the adults go about making their ice cream purchases. Adults aren't supposed to like these things, right? They like mushy stuff and vegetables.

Mia gets a sundae cone with plenty of chocolate and nuts, paying for hers and Draco's.

"Mmm, this is really good," she says between bites, offering Draco a taste from the cone.