1) Will this cause that child with the power of attorney to be held responsible for those credit cards?
No, a POA is not personally liable for the debts of the person for whom they have a POA.
2) Should credit card insurance be taken out on these credit cards for the widow and/or their children,
no matter what it costs?
I am uncomfortable with the "no matter what it costs" phrase. If the cards are in his name alone then it is too late for this anyways.
3) Should a death certificate be mailed to the credit card companies?
Yes. Go with the probate rules on this one for timing. Typically one should not be increasing debt on a card for which the owner is no longer living. However, if you do his estate is liable. If her name is on the card as well then she is also liable for the charges.
4) What should I do and who can help me?
Depending on the value of his estate you may need a probate. The personal representative of the estate is responsible for this and can hire a probate attorney to help. You might also consider contact the Army. Your MIL may be entitled to some survivor benefits for health insurance, etc depending on the nature of his retirement package from the military. If he was a member of the VFW or American Legion start with his post. Many posts have a VA representative stop by on a weekly basis.