If the sliders are profitable to sell, who cares? A sale is a sale. I'm with you - get people hooked on the food any way you can, they'll pay full price next time. It's not like you are going to have repeat customers who only buy a 2.50 slider. But even if you did, as long as it costs less to make than that (it should) who cares?
If not, you need to adjust your prices so you make money on the slider. It doesn't have to be a huge money maker, but you don't give them away for a loss.
Especially at fests, or anywhere with multiple vendors for that matter - lots of people like to have a small taste of a lot of different places' food. They don't want to have 10 choices and have to settle for just one because nobody sells small sample portions.
Keep them on the menu - it will put your food in more people's mouths, letting more people become customers for next time and that is the key to success.
Cat Daddy's BBQ : 22.5 WSM / PBC / Weber OTG 22.5 Cajun Bandit / Smokey Joe Gold