My kittens had a sticky ssubstance in their eyes as well. It started as a clear gelly like look and their last day, it was a yellow colour. This could mean they have an upper respitory infection, which, depending on how bad it is, can prevent the kittens from breathing properly and then the breathing can affact how well they can nurse. If it isn't taken care of immediately, this could spell disaster for your kittens, as it did with mine.
I would always be careful taking small kittens into the vet, but because the substance in their eyes has a color to it, I would recommend either taking them to a vet or see if you could get a home visit.
Make sure you moniter if they are nursing and how well, their weight and make sure they have a warm enough environment.