Live black worms are good for young clawed frogs, unfortunately for me, black worms cultures I'd get had leeches in them. I heard this is common with black worm cultures but I'm not sure.
I used to feed mine reptomin as well, they do sell 'baby' sticks which are smaller than the normal kind. You could also feed this frog mysis shrimp as well (the PE Mysis brand are large enough for juvenile xenopus), or you could just cut up small nightcrawlers with a scissor.
There are also sinking pellets on the market too, like HBH Frog and Tadpole Bites.. my frogs would never touch the stuff though so your mileage may vary on that product.