Make sure that you attach a wheelchair request to the reservation for the person who uses a cane. That will allow that person + one additional member of your party to preboard, before all other passengers. Each child 6 years and under can board with one other family member, under Family Boarding, which is between the "A" boarding group (the first 60 or fewer customers to board, after preboards), and the next group, which is the "B" group. Your 9 year old son could board with the wheelchair passenger, and the 3 year old with an adult in Family Boarding. That leaves your 9 year old daughter and two adults left to board. You could purchase the "Early Bird" option for those three, which usually prices at $25 per person, each way, and which automatically checks you in with a preferred boarding number; or, just make sure to check in EXACTLY at 24 hours before scheduled departure. The EB option would be a good choice for you, especially if you don't fly often, and you want to make things easier. The sooner you purchase the EB, the better the boarding number you receive.
When you arrive at the airport, just let a skycap or Southwest agent know that you need a wheelchair. There should be a tip to the person who pushes the wheelchair to the gate, just like you tip skycaps for curbside check-in or baggage handling.
This can be a lot to handle. You can call Southwest, and they will gladly help you with all of these requests.
Enjoy your trip!