How can I ensure that my estate isn't eaten up by debts and back taxes owed by my heirs?

An elderly widow has placed her house in an irrevocable trust with her four children as beneficiaries. One child is deep in debt, including state and federal taxes. How can she ensure that his share of the estate isn't seized by creditors or tax authorities when she dies? Can she bypass him and make his son (her grandchild) the beneficiary instead?