I will give you one answer, but this is so sensitive and I only answer as it might be good info to discuss with your physician and then your children. If one holds their breath and bears down, as in having a somewhat forceful bowel movement, it is possible, in susceptible persons, to dislodge a clot, or create an irregular heart rhythm and other related changes to the heart. It is called a commode coronary. The holding of breath and bearing down is called the Valsalva maneuver and this maneuver is used on purpose for some medical situations.
This is not to say this is what happened and again I repeat, I only write this as it is possible it may help your children.
I looked for all the clues I missed in Don's brain tumor and as a registered nurse, I beat myself up. In the end I was the one to insist on a brain scan, but it did no good for me to feel, 'what if'? Please, please don't take on anymore pain, but I do know that answers help and the idea it might prevent further harm help, too.
I know you asked CLKD. Hope you don't mind my chiming in.
If I can help in any way, please feel free to pm me.