After the short answer, plenty of long ones are now available, particularly from pediatric medical websites. But the main points of all of them seem to be the same:
- Start by avoiding harder to digest foods, like fats and meat protein and dairy.
- Use foods like the BRAT list as a starter, not as a strict, restrictive regimen.
- Take advantage of the food already at hand, that provides the same benefits, if you don't have the exact items on the list (bananas and applesauce, for instance, provide soluble fiber and electrolytes).
I ended up eating a fruit smoothie, some gluten-free toast, grape juice, and some almond-milk rice pudding. Plus tea (because one site added a TY to BRAT, allowing both yogurt and tea). Between naps, and big glasses of water.
Next time I may just call my mother, for the same advice plus sympathy. The internet may be handy, but it's darn short on sympathy.