You can check out this website:
http://www.medcalc.com/pedidose.html
but you would have to know for each specific medication what would be the appropriate weight-based dose. For example, tylenol is 10-15 mg/kg/dose, plug in the weight in the website and you'll get your dose.
To get your drug/dosage info i dont know of any websites, but the Harriet Lane Handbook is a great and widely used peds drug guide that will give you all of that.