One hunt, two very different sets of legs and eyes, and somebody usually ends up either bored or in tears.
Hide the youngest child's eggs at ground level, in the open, close to the start point. Hide the oldest child's further out and higher up. They end up hunting the same event without ever competing for the same egg.
Asking an older sibling to help hide, or to help a younger cousin find eggs in a section they already searched, turns a competitive hunt into a shared one, and an older child usually enjoys being the one with the secret more than winning outright.
Each child hunts only their own colour. This removes every dispute about whose egg it was, on sight, without an adult having to arbitrate mid-hunt.
Enough that the hunt lasts ten or fifteen minutes and not so many that finding one stops feeling like an achievement. A dozen or so per child is common, but there is no correct number.
Stand near their section and narrate temperature — "warmer, warmer" — rather than pointing. They still get the moment of finding it themselves.
Part of our guide to Holiday hosting with kids.