It isn't all that big

There are basically two concourses (plus a tiny Concourse C), each of which is just a long line of gates. The only moderately difficult part of making a connection would be if his arriving flight was in Concourse A and his departing flight was in Concourse B (or vice-versa), in which case he would have to walk towards security to the food court and then walk over to the other clearly marked concourse.

I don't know any 13-year-olds so it is hard for me to gauge what they are capable of, but a connection in Midway is much easier than in O'Hare or Atlanta (but harder than in a small regional airport, of course). The link has a map of Midway gates.

Click here for link at http://www.travelnerd.com/airports/midway-airport-MDW/termin