Nha Trang is an easy destination for families, largely because everything sits close together and the sea is calm for most of the year.
Which beach. Tran Phu is the most convenient, being central, shallow in stretches and never deserted. For shallower still, Doc Let in Ninh Hoa lets you wade a long way out without getting deep — at the cost of an hour in the car. Hon Tam works too if you want to fold in a short boat ride.
When the sun bites. Between about 11am and 2pm, outdoors is no place for children. The Institute of Oceanography is the best indoor fallback: living tanks, a whale skeleton, and cool rooms. The park complex on Hon Tre has an aquarium and a water park, enough to absorb a whole day.
On pacing. The common mistake is booking a full-day island tour for day one. Boats, sun, salt water and constant climbing in and out tire children faster than adults expect. Keep the first day light, put the island day mid-trip, and leave one afternoon empty.
What to bring. A long-sleeved swim top beats sunscreen for practicality, plus a wide-brimmed hat, sandals with heel straps for the rocks at Hon Chong, and water — heat dehydrates children quickly and they rarely say so.