Maui's hazards are specific and mostly avoidable, and they are worth saying out loud to children rather than assuming.
Shorebreak. Mākena's Big Beach and the main stretch of Baldwin both dump waves directly onto a steep sand face with no reef to soften them, and both produce spinal, shoulder and collarbone injuries in adults every year. The rule is simple: if the wave is breaking on the sand rather than out on a reef, nobody plays in it. Swim at Kamaʻole, Ulua, Keawakapu, Kapalua or Nāpili.
Rip currents and unguarded beaches. Many of Maui's best beaches have no lifeguard, including Nāpili, Honolua and Kapalua. Swim where there is a tower if you can. Hawaiʻi's drowning statistics for visitors are grim and they concentrate among people snorkelling alone in unfamiliar conditions.
Flash floods. The waterfalls and pools on the Hāna side respond to rain falling miles upstream in weather you cannot see from the pool. The Kīpahulu pools at ʻOheʻo Gulch are closed to swimming more often than they are open, for exactly this reason. Obey the closure signs, and get out of a stream if the water darkens or starts carrying leaves.
Sun. You are at 20 degrees north and the trade wind will lie to you about it. Mineral-only sunscreen is a legal requirement in Maui County, zinc oxide or titanium dioxide, so pack it rather than assuming you will buy it. Rash guards do more for a child than any lotion. Reapply after every swim, and remember the tops of feet and the backs of knees, which is where the holiday-ruining burn happens.
And the altitude one. At 10,023 feet the air holds roughly a third less oxygen. Children complain of headaches and tiredness before adults notice anything. If someone is unwell at the summit, the treatment is to drive down, immediately, and it works within minutes.