If it will save a meltdown, I don't think it's a bad thing. That said, we keep expectations of toys LOW while we are there and before we go. We have g'parents sending money for the kids to spend there, but it never goes far. We usually do one or two days where we even enter a store and buy one or two things. We don't buy balloons or anything like that, my kids don't even ask! And then on our last night, I usually go pick out an ornament and/or a picture frame or something with the year on it so we have something special to remember THAT trip. Personally, I wouldn't be buying ANYTHING if my kids were pitching a fit over buying something all the time though. It's not like a Disney vacation is cheap! I want them to appreciate that their biggest gift is simply BEING there. Anything else is extra and only for kids who are acting like big kids and being grateful!
Target has a ton of princess/cars stuff in the dollar spot right now - I just picked up a bunch for stocking stuffers.