Some schools provide DCPS-sponsored aftercare, others use private vendors. Usually, as long as you sign up by the deadline (usually in the spring around when the school registration deadline falls), they can account for your child and make sure they have enough staff on hand for the aftercare.
The waitlist usually comes into play for kids who get called off the school's waitlist later in the summer or into the next school year.
Here's the link to the page on DCPS aftercare and the schools that are using DCPS-sponsored aftercare this school year:
https://dcps.dc.gov/afterschool
Important to note that aftercare for some of the DCPS-sponsored programs doesn't start until the 2nd week of school. DCPS will post a list to the link above with start dates for aftercare at specific schools later this SY.