In a large, heavy pot, add potatoes, onions and garlic. Have a grandchild add enough water to cover all, plus another inch. Bring to a rapid boil, then reduce heat to a slow simmer. When a grandchild announces that potatoes pierce easily with a fork, drain water from the potatoes. Have a grandchild add the yogurt and butter, and mash until sufficiently blended. Salt and pepper to taste. Serve immediately.