First off..its never good to have frogs in the same tank with another species. Frogs are easily stressed out by other inhabitants beside the fact that each species has its own toxins and they will slowly be poisoning each other. Once the tree frogs are in an enclosure by themselves and they settle in and feel secure they will start eating. They will prefer crickets that are active and moving. They will only eat at night when it is completely dark so sorry you wont be able to watch them eat just yet. Count the crickets you are putting in and do a head count the next morning. Also look for frog poop that is always a good sign they are eating. Why don't you post a picture of their enclosure so we can see if there are any environmental concerns. What is the temp and humidity as that too will have a big influence on if your frogs will eat or not? When I first start with a new frog I use paper towels as the substrate so I can make sure all is well with eating and pooping. Then once I'm sure the new frog is healthy and eating properly then I add the substrate of my choice. Check out the care article to make sure you've got everything covered for proper care for your new frogs and give them time to settle in. Sometimes they get stressed just because we're fussing with them too much. They need to be quiet, dark, and alone. Once you've had them for awhile then they wont care that you're watching them eat. Right now they see you as a large predator. Eventually they'll see you as the food God.