The website you want is
www.iflyswa.com, which you've already visited if you bought a ticket. Just go to online check-in and input the info it asks for from your purchase confirmation.
There are no seat assignments. What you will get when you check in online(as far as 24 hours before departure) is a boarding pass, without which you cannot get through security at the airport. The boarding pass will have a group letter on it: A, B, or C, which corresponds to the signs above the gate door at the airport. When you arrive at the gate, get into the corresponding line; your line will start moving when it is time to board. Once on the aircraft, sit in any open seat you wish to, as long as it doesn't have a "reserved seat" placard in it (those are situations where someone has purchased two seats for one person, usually due to the person's size.)
If you have a 4 yo, you will normally be allowed to preboard, which means that you would look for the sign for the preboard area. Parties preboarding with kids may not contain any adults other than the children's parents. When you get on the plane, no one in your party may sit in an exit row if you have preboarded. It is a good idea to go ahead and check in online anyway, even if you expect to be able to preboard.
You need to be at the gate no later than 40 minutes before scheduled departure if you have an A and want to use it to advantage. If you arrive less than 15 minutes before scheduled departure, your seat may be gone, as they consider you a no-show if you arrive after that point, and will give your seat away if there is a standby passenger who wants it.