Hi Tracey .. a warm welcome to Silkysteps
I googled your question & came on a very nice conversation if a few years old !!
What do 
lets eat
Going by the answers there I'd say if the froglets are too big to be feeding on a piece of floating meat, a chunk of dog food is common I believe .. it may be an idea to move them outside with some damp bog wood with many minibeasts/flies & small worms in !!
You can buy frog/amphibian food from
CornishCrispa Company .. & have it on good authority that they are very helpful, so it might be worth an email to them to ask if the worms are suitable for froglets.
Good luck & let us know how they do.