I agree with Hawk1
Ditto.
X10 is great fun as well as useful software/hardware but should
never be depended on if life or limbs are involved. This is particularly true if X10 macros are involved that require a computer to be running. Any X10 door alert you use for this application should have a non X10 backup.
A pool cover should be used any time the pool is not in use for any extended time. Also a float alarm for the pool is a good investment as well.
I don't know the kids ages but I strongly recommend swimming and water safety lessons for all including infant classes as early as possible.
I think a simple local door alert that beeps are dings when the door is opened till reset
along with the awareness of all the adults and older children in the house may be your best approach.