Why are your children getting red small sores and blisters on their feet and hands after being in the swimming pool?
Abrasions from the bottom and sides. If they swim a lot they'll form callus' but they are a pain at the beginning of the season. Pool shoes will help prevent them. I assume you are talking about the tips of the toes and/or heels and finger tips.
The pool has either a bad, rough plaster finish possibly caused by a bad acid wash or bad prolonged water chemistry. If there has been no adjustment to the pH or alkalinity of the water then there salt stalagmites growing onto the plaster. If the plaster has been acid washed sometime in the past it was left rough by the acid. The job should have been completed by power sanding the plaster smooth. If this is a public or simi-public pool contact your local health athaurities. This is not a safe pool!! If a private pool stop using it until the problem has been corrected. If the sanitizer levels are not within parameters you can run the risk of an infection.