For example, it may be against the familys religion or culture to eat particular foods; some religions believe that certain animals are sacred so eating the animal would be extremely offensive. This is important to respect because if a practitioner tries to feed a Jewish child pork, for example, when the parents find out they will be extremely offended that the practitioner went against their religion, as that is what they believe in.
It may also be a personal choice; the parent or carer might just prefer their child not to eat particular foods. For example, in my setting one boy is not allowed to have sweets or chocolate because the parents have decided they dont want their child to have a sweet tooth. Therefore the parents have been respected in their decision, because again they may get offended if the practitioners decide not to listen to them.
Another, very important, reason a child will not be able to eat something is if they are allergic or have an intolerance to it. Therefore, if a child with an allergy or intolerance eats something containing the particular food it would mean the food will cause harm to their body, and sometimes very severely. For example, another boy at my setting is severely allergic to nuts; he must not come into any kind of contact with any kind of nut, because it will cause his throat to swell up and stop him from breathing. As a result of this any food that is provided for their snack must be approved by his mother, and if there is anything new the practitioners do not risk giving it to him, as they are not absolutely certain if it is completely nut free.