Vitamin D3 is called the sunshine vitamin because it might be synthesized in our skin when it is exposed to suffcient sunlight. Vitamin D3 that is obtained from food sources works equally good to support bones and teeth health.
For infants who are usually voluntarily kept away from direct sun exposure, the only way to supply them with enough vitamin D is through breast or formula milk. Adding a vitamin D3 supplement to their diet might be useful to be sure that proper amounts of vitamin D are supporting healthy bones and teeth formation.