I have definitely had times where I have had characters look 10 when they needed to be look 5. Sometimes it can be hard to figure out why a character looks the wrong age and even harder to fix it. Here are some pointers and things to keep in mind when changing the age of your child characters.
First of all, you need to consider the size of the face. Look at photos of babies or, even better, draw babies from life (you'd better be quick - they are squirmy little suckers!) But you will soon notice, that compared to adults, their chins and noses are quite small.
In the above diagram, all the figures have the same head placed on the same body. The body is just smaller each time (I also made the neck a little shorter each time.) By the 3rd figure, the body was getting too narrow, so I widened it a bit. But you can see by just changing the head/body proportions you can go a long way to changing the age of a character.
When you pair that with the facial changes discussed above, you can really alter the age of characters. I hope this helps.
Good luck and happy drawing!