Up to the age of five, sometimes longer, boys and girls wore the same clothes. This was probably for practical reasons as much as anything. Children of this age should wear a basic shift and a near floor length tunic or dress, these are very basically shaped. Kids can get away with no sleeves, but if you put them on don't make them too tight as it's really hard to get their arms through. The tunic can either be loose or laced at the side. The bonus of making a tunic is that it will fit your child for a whole season, unlike a dress. A coif can also be worn. I would suggest very generous hems otherwise you will be making new kit every second event. Full length coats are a good idea for colder events and can look really cute as do hoods with liripipes!
Now I know that some people have an aversion to putting little boys in what they see as a dress, but some of the men's wear is hardly normal by today's standards. After all, most people would say medieval men wore tights. Please put boys in a "dress" until at least they are out of nappies, as hoes and nappies don't mix. I would suggest that it doesn't do them any harm at all, but if the child objects then that of course must be taken into account. However cute you think a two year old in hoes is, it isn't authentic!