Children have less body mass, so whilst someone who is older is more likely to have a heart problem due to it, the children are likely to be affected simply by the temperature much faster, and if the young girl was first in and had trouble then that's one, then the father goes in and possibly the immediate shock causes him problems, then the boy has two people in trouble.
It's unfortunately not at all uncommon for multiple people to die trying to help someone out in cold water

it's one of the reasons IIRC the official advice from pretty much every emergency service is to not get in the water yourself unless you are certain you will be ok, and there is no other choice (and why from memory Police officers are expected to wait for properly equipped personal if they have a report of someone in the water, but cannot see them).
The design of the pool sounds, like many at holiday destinations, to look good but with little thought to safety, especially if there was no life guard around, and no way to assist the person in trouble out of the pool without going in yourself.