Sanur is usually the first beach families try, and for good reason. An outer reef keeps the water calm almost all day, so there's no real swell to worry about, and a long paved beach path runs the length of it, which makes it easy to push a pram or chase a toddler without worrying about traffic. Low tide exposes a lot of sand and some patches of reef and seagrass, so aim for mid to high tide if you want clear swimming water, and pick a spot near one of the beachfront warungs where you can rent a sunshade.
Nusa Dua has some of the calmest water on the island because the bay is almost fully enclosed by reef, and the sand stays soft and fine, which toddlers tend to like for digging and building. Jimbaran's curved bay is another good option: the slope into the water is gentle, the water is usually flat by mid morning, and it's a nice spot to let kids play in the last hour before sunset when the heat has dropped. Both areas have calmer mornings than afternoons, so if your child naps in the afternoon, plan the beach time for earlier in the day.
Skip the open west and south facing beaches for anyone under about seven: Kuta, Legian, Seminyak, Canggu and Uluwatu all get real swell, shorebreak and rip currents that catch out grown adults, let alone small kids, so they're better suited to older children learning to surf than toddlers wading. Wherever you go, there's rarely a lifeguard watching your specific spot, so keep young children within arm's reach in the water and treat any flags or posted warnings as real. Shade is scarce on most beaches, so bring your own umbrella or book a morning slot at a beach club with a shallow pool as a backup on choppier days, and keep sun hats and reef shoes on hand since the sand and shallows can get hot and occasionally have sharp coral bits.
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