Many conditions have an effect of creating drowsiness or fatigue.  Still others without this effect as a part of their condition, may experience it via medication they may be taking. 

If there is a desire from the player to provide assistance, or if it is preferred by them to remain engaged throughout the session.

• Take regular breaks and when applicable encourage players to get up, stretch, and hydrate.
• Ensure that engagement occurs more frequently with the child even if it is not within a turn. 
• If they provide a dietary restrictions list,  avoid those restricted foods. That can also include sugary or caffeinated beverages.
• Because fatigue also can affect concentration, allow for additional time on the players turn, and encourage/assist with notetaking.
• If the need calls for it, you can also shorten session times, group fewer sessions together, and avoid complex session components.

Much like in the case of if a player experiences increased absence or tardiness, you can provide reading materials, puzzles, and components for home use, so that the player can be working toward the end goals of the group on their own time and in their own environment without the structured limitation of a gameplay environment or make use of some form of free online communication tool (google hangouts, roll20, fantasyrealms, etc).