You might consider removing the tree fern completely. ( actually, it could scratch their faces )
If you want a dark background, you could paint the glass on the outside? This way you can keep the in glass clean on the inside.
Just think how contaminated the tree fern will be after the frogs have peed on it a few times!! This is exactly how frogs get sick!

You could feed the crickets on the substrate AND in a clear class bowl w/ about a 3" side.
-clear so the frogs can see the crickets through the bowl
-3" so the crickets can not climb out

After they learn to use the bowl you will not longer have to feed on the substrate.
They will get hang of the bowl very quickly. However, be sure you see them on/in the bowl before you discontinue feeding on the substrate.