We are talking hatchling dinosaurs and maybe some of the smaller species. You are correct, when the word dinosaur is mentioned, we tend to imagine gigantic animals. But dinosaurs filled a huge variety of ecological niches; while there were many gargantuan giants, there were also a large number of dinosaurs that were very small, even as adults. Compsognathus (the "compy" of the Jurassic Park films) was only about the size of a crow as an adult; the iconic velociraptor was smaller than a modern turkey. It would have been very easy for a large frog with a massive bite force to feed on the offspring or juveniles of these species. Since there are modern frogs that will occasionally eat birds, it's not unreasonable to imagine one eating a small bird-sized dinosaur.
Thanks a lot for the quick lecture. It was quite expository