Each member has their own policy
In Switzerland, health insurance is individual: there's no single "family contract" for the basic cover. Each family member has their own LAMal policy with its premium (reduced for children and young adults). You can still group the family with one insurer for convenience, and many insurers offer family discounts on supplementaries.
Insure your baby from birth
The newborn must join basic LAMal (within 3 months of birth, retroactive to birth). Important tip: taking out the baby's supplementaries before birth (or very early) avoids the health questionnaire and guarantees access to supplementaries whatever happens. It's the best time to insure a child: in perfect health.
Which supplementaries for children
The most useful: dental cover (anticipating often-costly teenage orthodontics), an accident/sport supplementary, and sometimes alternative medicine. Subscribed early and healthy, they're cheap and guaranteed. For the base, prefer the minimum deductible for young children (frequent visits), and compare children's premiums which also vary by insurer and canton.