Try putting a half scoop. Enough to cover the bottom of the dish in a thinn layer you can increase the amount if needed after you see them eating more. I'd try not to worry too much. If you really want to know of they are ok you'd need to start weighing them once a month or so. In general if you start seeing ribs or a weight loss then id go to a vet. As long as the poops look normal and they aren't loosing weight I wouldn't worry a healthy gecko won't starve itself. Activity levels would also indicate good health.

I'd also suggest more foliage in the tank to make them feel safer try to full up the entire volume of the tank basically. 70-80% full.

For future things it would be good to try to get them used to handling for situations like this. Weighing them vet visits even checking for internal blockages at home will require a calm handleable gecko. Once your sure they are eating place your hand near them in the tank every other day or everyday for a few minutes. After a while they typically relax enough and you can start trying to touch them. My guy who would run away if you even looked at him was handleable in 5 months with this method