Next time they do this, take a pretend taste and say, "YUCK!!!!!" and make a terrible face (sticking out your tongue is always a nice addition). You might thing you'll hurt their feelings, but you might be surprised at how funny they think this is. (Of course, if your child is particularly sensitive, and appears hurt by this, then you can remedy the situation by claiming a bug in your portion and redoing the taste test.) They'll take it back to their "kitchen" and try again. Continue with the yuck as long as they are laughing. Don't be surprised if they start bringing you new combinations like "Scrambled Worms with Eyeball Sauce." That's a good time to start praising their cooking skills again. Mmmm.
You can change it up by screaming, "YOOOWWEEEEEEE!!!!! THAT'S HOT!!!!" I have gotten lots of giggles from that tactic as well.
If you always give your kids the same response, they'll assume you're not paying attention. Pay attention. Make each time different. It doesn't take a lot of effort and the smiles will be worth it.