Probate costs will be grossly inflated. All of the Pension and IRA assets will have to be distributed within five years of their deaths accelerating the income tax that otherwise could have been deferred. The courts will have to step in to appoint guardians for their children. Mark and Mary will not be able to choose who raises their children. Because minors are not allowed to own property or handle their own finances, a guardian will administer the entire proceeds of the estate under court supervision.
Guardianships are expensive and restrictive.
Believe me, you don't want to go there. Perhaps as bad, as soon as each child reaches the state age of majority, they get all the remaining funds. To put it kindly, this is often not a good thing for young people. Too much money too young with no guidance attached could be a disaster. Name a guardian to raise the children in the event of the death of both parents.