This is the decision parents postpone longest. Fees and Executors get dealt with quickly; the question of who would raise the children if both parents could not is left open for years — which leaves it to be settled by other people at the worst possible moment.
What the appointment does. A Guardian named in your Will takes parental responsibility for children under 18 where no parent with parental responsibility survives. They decide where the children live, which school they attend, and everything else a parent decides. Note that a surviving parent who holds parental responsibility will normally continue caring for the children whatever a Will says, which matters in separated families.
Without an appointment, nobody has automatic authority. Relatives must apply to court, the court decides on the children’s welfare, and in the meantime the children wait while two sides of a family potentially disagree.
Choosing well means asking about suitability over fifteen years rather than who loves them most. Consider where the Guardian lives and whether the children would change school; whether they have children already; their energy now and in ten years; their values on education, religion, discipline and money; and their stability. Grandparents are the instinctive choice and often the wrong one on age grounds — many families name a sibling first with grandparents as fallback. Always name a substitute.
Ask them first. Nobody should discover the role after a death, and a person too polite to decline can simply renounce later, leaving the family where it started.
The forgotten half is money. Appointing a Guardian says nothing about funding. Without planning, children inherit outright at 18 — rarely what parents intend — while the Guardian funds a bigger house and years of school runs from their own pocket. A trust in the Will solves this: Trustees release funds for maintenance and education, with capital passing at 21 or 25, or in stages.
Keep the Guardian and the Trustees as different people. One asks for what the children need, the other considers it. That separation protects everyone, particularly the Guardian.
Add a letter of wishes covering schooling, contact with both families and anything you would want your children told. There is no perfect answer here, only a considered one — and it can be changed whenever life does.