In other words, make-your-own Pedialyte (or Gatorade). I discovered a variety of links online to make this, and found two favorite blends. Depending on what you have in your cupboard, either of these will help your little one (or not so little one) stay hydrated when sick.
EITHER: 1 packet of Koolade (any flavor) & 6 Tbsp warm honey
OR: 1 packet of Jello (any flavor)
2 quarts water, warm (just to help everything dissolve)
1 t baking soda
1 t salt
Mix everything together in a pitcher and store in the fridge for up to 7 days.