Conditions that can limit, delay, or terminate attendance
Keeping to a regular schedule may be difficult. Sessions can be designed to be only shorter increments, but chained together so that they can accommodate a group dynamic that can handle longer play times. Determine a story that can explain the occasional disappearance of a hero between sessions. Suggest that they are researching another angle to a mystery, holding back to ensure that a camp is secure, listening for a transmission, or decoding a program. Anything that explains an actual absence can be rolled into the narrative so that gameplay and experience isn’t slowed.
It is also helpful in this situation to recap the prior sessions at the start of each new session in order to keep everyone aware of what is happening, while not calling out the reason.
While it may not be ideal to the gaming environments dynamic, consider options for remote play, or hybrid remote play using online technologies such as roll20, fantasygrounds, etc. A purely remote play session would allow all children to be located within their home or location of choice, while hybrid sessions allows for a single player to remotely connect into a device and interact with a seated group.
Additionally you can encourage children by providing them puzzles or exploratory story lines that they can take or work on at home. This will encourage them to feel like they are still a part of the team working toward an overall case or adventure.
If the issue is a matter of the availability to travel, you can consider the use of implementing technology such as online tele or video communication. Most smart devices allow for it, but there are also a host of applications that can be used.