Writing young characters can get complicated. This is because there are a large variety of ages, behaviours, and sizes of young characters. To write such a character you first have to figure out their age and gender. With older characters age does not matter, but with younger characters, especially those in their early childhood, behaviour can change greater even after only one year.
Now since you have figured out your characters age it is time for you to do some research. You can do this either by looking into the psychology of children that age or by observing children that age. If you decide to observe make sure that you aren’t doing it in a creepy way that could cause people to think that you are a stalker or a pedophile. This is best done in a passive way throughout your day to day life.