death of sow after farrowing
This is an unfortunate incident and I am sorry that your children had to experience it. From the information provided, it is difficult to determine what could have happened. I am not sure of your location, so that also makes it harder.
If you live in a area where it is hot and/or humid, heat stress can be very difficult for lactating sows to overcome. Pigs do not perspire very well, and need other means to cool themselves.
Also the farrowing process is a very stressful time for the sow. Research has shown that around the time of farrowing sows are more prone to conditions such as heart attack, and other physiological disorders as the sow transitions from pregnancy to lactation.
When pigs die suddenly from no apparent reason, it is always a good idea to have a veterinarian to do a post-mortem exam. The veterinarian may be able to determine what the animal died of. You can then use this information to reduce the chances of it happening in the future.