It's covered in Sophie's lip gloss - the little container in front of Suzi is Sophie's box of play make-up that she hides in another bag and hangs up on her dresser. Suzi found it.
This is Suzi realizing she's caught. Very distraught, isn't she? It must be the regret causing her to laugh so maniacally.
And yes, that's one of Sophie's belts that she's wearing as a headband.
Rotten little monkey!