What about springtails?
I had a froglet so skinny I thought he was going to die. He was being bullied in his tank. So I moved him out to his own 10g, and fed him springtails. He was so weak he could barely move, so I'd drop just a few springs at his feet. He'd eat a few of those before they scattered off. Then I'd drop a few more, and so on, until he had a decent meal. I did this every night for, I dunno, maybe a week, until he was able to take small flies.
Then I'd just give him a few at time. When they are already stressed and skinny, you don't want them stressing more with too many flies crawling around.
He lived and now he's an active, normal sized, frog.