Buying two or more of the same pattern for your kids because they are not the same size, don't, only buy one! If the envelope has all the sizes one is all you need, if it does not go ahead and buy one for the next set of sizes.
Here is why,
I learned this only by trial and error and making the mistake of cutting out a pattern that was the wrong size. With all my boys their sizes change often and I needed a way for my pocket book not too change size. By tracing the size/s you need, you can keep the master pattern for a infinite number of uses and not just one per envelope. Rarely does the pattern get the waist and leg length right so this allows for adjusting the traced pattern with out messing up the master. I keep traced individual sizes that I have altered on file to keep track of what works for my boys and what alterations didn't. This is also a easy way of expanding your pattern collection with different styles from the same base pattern.
You WILL save money and some sanity by using this method.