Before I took the seat apart I would try a shop vac with a tapered nozzle and try to suck it out while rapping on the buckle with something to loosen it.
Depending on how far down it is, a pair of forceps might work. They’re good enough to get a bean out of a kid’s nostril 
paper clip.
bend as needed.
If you can see it, take a paper clip and put a drop of supper glue on the end of it. Then touch it to the edge of the quarter. Wait for it to dry and pull it out.