It is sad Bob but this is normal life for Mum and chicks. If it hadn't been the swimming pool it would have probably been something else in time. The mums here do not seem concerned as long as some survive. The hardest is when a hen only hatches a single chick and it dies. Then you can see the effect it has on the hen.
There is a tendency for humans to believe they can save them all and keep them safe. It's not a healthy view for the keeper or the chickens and often ends up with imprisoning the chickens in the belief that this will keep them safe. It doesn't and the quality of life is pretty appalling in many of the keeping arrangements one can see on BYC. Yes, it is better than most egg production keepinng arrangements but is a bit better good enough, especially when one thiks that the majority of breeds kept by backyard keepers only exist to cater for the backyard keeping craze. The keeping of such breeds has virtually no impact on the large meat and egg production businesses because these breeds are not used in such concerns.
For all the guilt and sadness you may feel, you've done the right thing in both letting Sydney sit and hatch and then allowing her as much freedom as possible in your circumstances. For a short period mum and chicks lived as close to a natural life as is possible in a backyard environment and that is the best gift of all.
Every hen here apart from Blue Spot has lost one or more chicks while still under their care. You will at some point I hope see the point at which mum stops full time care and the chicks pretty much abandoned to take care of themselves. This is hard to observe as well. For me it is harder than a death of a chick. The sudden transition from 24/7 care and mum ready to die for them, to getting pecked and bullied and pushed away by mum as well as the rest of the flock I've found heartbreaking.