With your CYA at around 50 ppm, an FC of 20 ppm would have around the same active chlorine (hypochlorous acid) level as 0.6 ppm FC with no CYA so is lower than most indoor pools that don't have CYA. You wouldn't want to drink the pool water in large quantities (that's why the EPA drinking water limit is 4 ppm FC, assuming 2 quarts drank per day), but it's not unsafe to swim in. Nevertheless, compared to your normal FC that is closer to 10% or so of the CYA level, the chlorine at shock level will oxidize your swimsuits, skin and hair faster, probably around 6 times faster or more. Again, this is something many people experience swimming in some commercial/public pools, especially indoors (my wife can attest to this).
So while not ideal or something you'd want to do long-term, it's OK for what you describe. If the kids sweat, then they'll likely smell more monochloramine on their skin shortly after they get out since there will be more chlorine capacity (FC) to react with their sweat and produce a chloramine smell. They may also smell a slightly stronger chlorine smell near the surface since chlorine will outgas faster at its higher concentration. It probably won't be very strong, however.
If you are concerned, then you can certainly let the level drop and pick up shocking again some other time.
As for chlorine loss rates from sunlight, for any given CYA level, the loss rate is proportional to the FC level. At 50 ppm CYA you might expect to lose maybe 30-40% of the FC level during the day though that depends a lot on the amount of sun exposure, latitude (angle of sun), etc. So the FC might drop from 20 to 12-15 ppm. At your more normal FC level of around 4-6 ppm, you'd probably lose more like 1.5 to 2.5 ppm FC per day. 3 ppm per day is on the high side and usually means something more than sunlight, such as algae, is reducing the chlorine level (unless the CYA level is low).
So while not ideal or something you'd want to do long-term, it's OK for what you describe. If the kids sweat, then they'll likely smell more monochloramine on their skin shortly after they get out since there will be more chlorine capacity (FC) to react with their sweat and produce a chloramine smell. They may also smell a slightly stronger chlorine smell near the surface since chlorine will outgas faster at its higher concentration. It probably won't be very strong, however.
If you are concerned, then you can certainly let the level drop and pick up shocking again some other time.
As for chlorine loss rates from sunlight, for any given CYA level, the loss rate is proportional to the FC level. At 50 ppm CYA you might expect to lose maybe 30-40% of the FC level during the day though that depends a lot on the amount of sun exposure, latitude (angle of sun), etc. So the FC might drop from 20 to 12-15 ppm. At your more normal FC level of around 4-6 ppm, you'd probably lose more like 1.5 to 2.5 ppm FC per day. 3 ppm per day is on the high side and usually means something more than sunlight, such as algae, is reducing the chlorine level (unless the CYA level is low).
This house now has a beautiful clean clear pool, with a completely de-stressed and confident owner. I can't thank you all enough. I have spent less $$$$ in the last 6 weeks on all chemicals than I did on each of my previous 5 visits to the pool store. 
