Hatchling (pinhead) crickets are ideal, but unfortunately they are hard to find. Only a few petshops carry them. I've tried raising my own, but it's not easy. They can be ordered by mail, but the shipping charges are insane.
I raise my own fruit flies (the flightless kind). I currently have some toadlets (Bufo woodhousii) that are doing great on a diet of dusted fruit flies. They have no trouble catching them. The flies are easy to culture.
In the immediate time frame, you could try using fruit scraps to attract wild fruit flies. Then you just have to figure out how to get them into a container with the toadlets. I keep a piece of cloth under the lid of my toadlet container to keep the fruit flies inside.
Overall, it may be better just to leave the tads and toads in the pond and observe them from there. If you have any exotic pets in your house, you run the risk of infecting the toadlets with foreign germs and then putting those germs back out in the wild (where they could infect the other toads in the pond). In general, it's not a good idea to keep animals captive and later release them, although I can certainly see the educational benefit to watching the metamorphosis process.