Many medications come in a variety of forms (e.g., infant drops, children's suspension, chewable pills), concentrations (the amount of medicine per unit dose), and are given based on age and weight. You can't just substitute one teaspoonful of the infant drops for one teaspoonful of the children's suspension (or vice versa). The amounts of medicine in each are very different--so if you are not careful, either overdosing or underdosing may happen. Medications should be somewhat pleasant tasting, easy to take, and hopefully not in large amounts or needed too often. Otherwise, kids won't take them, parents will struggle to get them in, and the health benefits will not be appreciated.
The Food and Drug Administration is aware of the importance of clear labeling for families, particularly with over-the-counter medicines. I personally never recommend giving acetaminophen for more than a couple of days in the recommended dose and frequency, without at least contacting a physician or reviewing the symptoms, to see if the use and the amount of the over-the-counter medicine is appropriate, and to determine whether the child needs to be seen. Most importantly, remind yourself that medications come in different concentrations, the recommended doses must not be exceeded, and dosing too frequently must be avoided. If there is ever a question about the proper dose or frequency, it would be important for you to speak with your child's health care provider.