All children 18 and under must be accompanied by their parent or legal guardian.
At the Church of the Ascension in Silver Spring (the Saturday venue), there are no plans for childcare and no nearby playground or park, but well-behaved children (accompanied by parent or legal guardian) are most welcome in the singing hall. They are free to sing or listen to the music as they wish. Sacred Harp singers believe that it is good for children to be exposed, actively or passively, to the sound of the music and the concept of community-style singing.
However, if one brings an infant or young child to the convention, it would be wise to sit in the back rather than front rows and to use ear plugs to reduce the chance of permanent hearing loss from loud sounds over an extended period of time. Experts believe (e.g. ref 1 and ref 2) that loud sounds (higher than 85 decibels) can permanently damage an infant's or young child's hearing. Decibel levels at large Sacred Harp conventions have actually been measured in the 100 decibel range and discussed with respect to hearing loss on a 2009 thread on the Fasola-Discussions discussion group.
The Lacey Woods Park (the Sunday venue) has a children's playground on site.