After leaving Eileen, Fred wandered through the crowds of people. After accepting a drink or two from the house elves, the redhead sat back and considered his next move. They were at the Malfoys', which clearly meant he needed to do something. Normally spiking the drinks would be his first option, but with elves doing all the serving he had to think just outside the box. Luckily, he was very good at that.
Passing through the crowds, he found a small corner where no one could see him. Pulling a niffler out of his pocket, he enlarged it, tucking it behind a potted plant. He felt a little bad about having under a sleeping spell, but he couldn't have them making a nuisance of themselves until he wanted them to.
Fred repeated the process several times, being careful to be far away from any of the drop spells as he released the spell and waited for the inevitable screams as the little rodents began to bite when they discovered jewelery.