From Southwest's web-site:
Assistance in the Airport and with Boarding
Upon arrival at the airport, please inform a Southwest Airlines Agent or Skycap at your first point of contact, either at the Skycap podium or the ticket counter, if you need an airport wheelchair and/or assistance within the airport.
Priority preboarding is available for those who have a specific seating need to accommodate their disability and/or need assistance in boarding the aircraft or stowing an assistive device. If a Customer has a disability and requires preboarding, the Customer should request a blue preboarding sleeve from our Customer Service Agent at the departure gate. The sleeve serves as notification to our Operations (Boarding) Agent that the Customer has a need to preboard. It's important to keep in mind that Customers who preboard may not occupy an exit seat. Customers with disabilities who have any other special needs related to boarding should speak with our Customer Service Agent at the departure gate.
Customers who need additional space to accommodate their disability may proactively purchase an additional seat to notify Southwest of their specific seating need so adequate seating onboard the aircraft can be ensured. Instructions for booking a second seat online can be found on our Customers of Size page. Customers may contact us for a refund of the cost of the additional seat after travel. If a Customer with a disability has concerns about the amount of space needed and does not wish to purchase an additional seat, the Customer should speak with the Customer Service Agent at the departure gate.
A small wheelchair that can fit down the aircraft aisle is available at every gate and will be used if you need assistance transferring from your wheelchair in to the aircraft seat. Each gate is equipped with a Passenger Transfer Kit (PTK), which contains a slide board and a transfer sling. The sling allows for two or three Employees to lift the Customer safely from his/her wheelchair into the small wheelchair and then into the aircraft seat.
For ease of transfer, the first two rows (at a minimum) on most of our aircraft are equipped with movable aisle armrests. Note: Our 737-800 Series aircraft do not have movable aisle armrests on the bulkhead (front) row of seats. However, all other rows on these aircraft have movable aisle armrests.
Our Employees are trained on assisting our Customers into and out of the aircraft seat; however, the Customer being transferred knows the best way for us to help. We encourage the Customer being assisted to direct the Employees in how best to perform the lift and transfer so everyone will be most comfortable, and so that we will have a successful transfer.
Information for Burbank and Key West Customers
Customers traveling to, from, or through our Burbank and Key West Stations board and deplane our aircraft via stairs, as these airports are not equipped with jetbridges. Customers who are unable or have difficulty climbing stairs will be assisted with boarding and deplaning by use of a mechanical lift. However, on some occasions, due to weather or other conditions that may exist, level-entry boarding may not be available.
For pre-boarding with children I believe the age is 4 or younger.