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Certainly these drugs or lozenges are not meant for use in small children without medical supervision. Please ask your mother in-law to desist from these practices and advise her instead to reward children with raisins, fruits, or just plain sugary treats occasionally like acid drops.
Daily use of too many lozenges can be bad for tender teeth of toddlers.
Long term use of cetirizin or benadryl also can cause blood disorders.