How does your fiance feel about this? Maybe since it's his parents that are doing this, they'd listen to him more so than you. Or maybe you both could sit them down and talk to them. Very direct and straightforward. "We don't want you feeding our dogs excess human food. We had dog treats for that...." etc..

They need to realize that the dogs are you and your fiance's responsibility and they need to listen to you two about their care. Or maybe they could join you at your next vet visit and you could have your Vet explain the problems with feeding dogs (Esp Toy breeds) excess human food. Maybe they'd listen to someone with a degree?

Good luck.