If they just cite at the end, it seems like that leaves the first sentences ambiguous to the reader. But citing the same source after every sentence does seem overdone. I suggested to one student that she state at the beginning of the para something like, "As Joan Doe describes in Work X (proper citation),...." then "Doe continues by noting..."
If they cite just at the end, the reader assumes that all the ideas in the paragraph are from the one source, and nothing is from the writer of the research paper. If all the ideas in the paragraph really are from the source, then they are letting the source take over the paper.
This is a good time to teach them, as you say, to write, "As Joan Doe describes...." They need to take command of the paper and the sources, not just plop a paragraph full of ideas from a source and then cite the source.