I read this in a sci-fi magazine in the late 1970s/early 1980s, though it might have been published earlier. The protagonist is an artist who accidentally wanders onto a beach stranded in time that contains people and animals from all times who never age. The beach is at twilight. Anyone can leave the beach at any time but they always return to the time when they entered.
The protagonist finds a trilobite and then encounters a group of children (who I think are from the future and came to the beach to flee political persecution). They have lived there for many (millions?) of years and are jaded and looking for novelty.
The children are very interested in the artist and ask him to sketch them, so he does. He doesn't realize that he's not in the normal world and says he won't be able to sketch them all before the sun goes down but the children laugh because the sun never changes position here. Later one of the children is killed when they encounter a Tyrannosaurus Rex and the boy taunts it.
In the end the artist leaves the beach and returns to his own time.