Sorry for your loss :/ As tomato frogs are a burrowing species coir would usually be considered the best but impaction is almost always a risk with a substrate. Although paper towels can be used and are safer, they don't allow them to burrow. It might be hard to compromise a 100% safe substrate and one that allows burrowing. You could try feeding them in another enclosure if possible but I suppose this may stress them out. Because sphagnum moss has long fibers that soak in a lot of moisture and therefore pack down well, this may be relitively safe and will help with the humidity requirements as well. It could be placed ontop of the coir.