Grandma brought the five-year-old some plaster blocks with little toys inside. Surprisingly, my kid loved digging out the toys. (This would drive me crazy.)
Of course you could get some plaster and molds and make your own kits. Once our initial supply ran out, we just ended up buying a 12-pack of blocks from Amazon (paid link). I like this particular product because the “bones” are also decent-quality plastic toys.
My kid spent around four hours (!) over two days digging out one set of “bones.” I regard that as cheap entertainment—and a nice break for parents. And you can integrate the project with some actual science education about dinosaurs and paleontology.
Here we’re watching actual scientists digging out actual dinosaur bones at the Denver Museum of Nature and Science.