First off, an inseam measures the distance along the inner seam of a pair of pants between the groin and the bottom of the ankle. To start, you are going to need a measuring tape and a pen and paper to record your measurements. 

To measure your child's inseam, have him stand up straight with his legs slightly apart. Be sure he is not wearing shoes. Socks are fine. Using measuring tape, measure from just below his crotch to the bottom of his ankle. Record the length in inches; this is his inseam.

If your kid is too active to keep still long enough to take a measurement, you can measure the inside seam of a pair of pants that fit him well.

Place the pants on a level surface and measure the inside seam from the crotch to the hem.

Things to Consider

It's important to note that the inseam length somewhat depends on personal preference. The following considerations may change the type and length of pants you choose to by for your child:

Shoes. If your child wears orthotic inserts in his shoes, you may want to be sure he wears the pair of shoes and inserts he'll be wearing with the pants to ensure the right length. If you prefer your child's pants to be a little on the longer side or folding slightly on the top of the shoe, measure the inseam from the beginning of the shoe, just below the ankle bone.

Shrinkage. Take into consideration the material the pants are made of. Some fabrics (especially natural fabrics such as cotton, linen and wool) tend to shrink more than others when washed. If you are worried about potential shrinkage, check what material the pants are made of and add an extra half an inch to your child's inseam length before purchase.

Style. Some pants and leggings are designed to come in cropped or three quarters of a length styles. If you’re unsure of how to measure the inseam of your child to fit these styles, try re-measuring the inseam and stopping the measurement at the point where you want the pants to end and see if the inseam matches the height and waist measurements that coincide with the sizing chart for the product.

If you still feel unsure of your measurements, most online store websites will provide you with inseam information near their size charts. Another trick is to read other customer reviews that provide more information about sizing and fit of the specific product. Customer reviews will often reveal whether the pants are going to shrink, stretch or fit tighter or smaller than other pants of the same size. It may take some time to sort through customer reviews, but they can be very revealing as to the exact fit of a piece of clothing. Finally, be sure to look for the store's online shopping and return policies, as nowadays you can simply return your purchases with a click of a button.

Source: kids fashion