The custom of disallowing the use of ‘Kitniyoth’ (soy or corn, for example) was never applied to infants nor to people who are ill.
Despite the fact that Kitniyoth for infants is permissible, the Mishna Brura states that it is preferable (but not mandatory) to avoid giving infants rice, buckwheat (kasha in Yiddish), or millet on Passover.
However, one must take care to keep Kitniyoth baby utensils or cutlery away from the general kitchen area. Any mixing or washing should be done elsewhere, such as in the bathroom sink.
We have indicated that Kitniyoth is permitted for infants – but that does NOT cover Chametz, or any food that was processed on equipment that was not properly cleaned for Passover. Such Chametz is strictly prohibited even for infants.
When we talk about Kitniyoth being permitted for infants, it means that it would have to be a homemade Kitniyoth, unless the Sephardic community has a reliable Hechsher on an infant product.
The Halacha (Jewish law) also suggests that separate pots, dishes and cutlery be dedicated to the Kitniyoth food.
Obviously, in cases whether a doctor insists that the infant must have the Chametz, the prohibition does not apply. In such a situation, it would be preferable (if possible) that a gentile should bring his own Chametz and feed the baby. If that is not feasible then a Jew would be permitted to feed the infant Chametz.
The upper age still considered an ‘infant’ depends on the child. Obviously, if the child can eat healthy food that is not Kitniyoth, that would be preferable. Usually, the Poskim indicate that two years of age would be the upper limit, but if a 2-year old is not ready to eat other food, then certainly the limit goes up as high as his or her Bar or Bat Mitzvah.