Conditions that cause pain
There are two types of pain that children may experience while playing in your sessions. Either persistent ongoing pain or rapid onset pain. Conditions that cause this can vary and are often supported heavily by specialized equipment the child will have in advance. Ensure that your environment has the capability of accommodating that equipment in space and access.
Certain pains may derive from things that occur from their conditions that standard first aid support can respond to, such as joint swelling, or bruising. In this case, it will also help to print out and have ready materials that support PRICE (Protection, Rest, Ice, Compression, and Elevation.)
One coping recommendation also comes from distraction. Reducing time when players are not engaging can help in creating that distraction. Preplan character actions, reduce monologues and exposition. When roleplaying focus more on interactions and back and forth dialogue. In combat preplan attacks to return your turn order and shifting it back to them. In describing combat actions reduce discussion of concussive battle actions and focus descriptions primarily to things that limit the hero to reduce their YP. For example, a hero can just as easily be described as “numbed for 4 YP” as they could be described as “hit for 4YP”. Remove a focus on painful interactions will help shift focus away from pain and thus assist in distractive efforts.
The opposite may occur in that a sedentary position or continued focus can generate pain. In these instances take frequent breaks, and focus on creating moments as touchstones to check on the player's needs.