Is it okay to mix childrens allegra & childrens tylenol (acetaminophen) to treat cold/pains on a 2 year old?

Yes but..... Tylenol (acetaminophen) and Allegra are ok together but Allegra is not useful for "colds" as it won't control the runny, stuffy nose unless due to an allergy to inhalants (e.g. Dust mite, pollen, pet dander). Current american academy of pediatrics recommendations are use tylenol (acetaminophen) for fever, aches but no antihistamines or decongestants for nasal symptoms as they are largely ineffective and can have side-effects.
Allegra & Tylenol (acetaminophen) This combination is quite effective and safe. Carefully check the dose. Too much tylenol (acetaminophen) can cause liver damage. If a child or an adult has repeated "colds", allergies should be suspected and evaluated. If the child has fever and cough a check up is important to rule out pneumonia or bronchitis. Play safe and don't over rtreat.