Knowing exactly what size clothing your child needs can be almost impossible at times as all shops seem to size clothes differently and they are often staged in years. I have a 3 year old that is already in age 4-5 clothes and a 2 year old that is still in 1-2, so often you cannot even rely on the ages specified. If your child is tall but slim you may find that trousers for his or her age are too short but the next size up is too big. If this is the case then I would strongly recommend buying adjustable waist trousers. These trousers are fitted with elastic in the side which can be tightened or loosened using the button attachments.
These trousers are great as they can often be adjusted 1 to 2 sizes allowing them to be used for a longer period of time. Although these are often a little more expensive then nonadjustable trousers, you certainly get more wear out of them.