I wouldn't give crisps, they're salty and fatty, and quite uncomfortable to vomit back up.
Dioralyte is an idea.
Here are some symptoms of dehydration in a toddler:
• Goes more than six hours without a wet diaper
• Has dark yellow, strong-smelling urine
• Is lethargic
• Has a dry, parched mouth and lips
• Isn't shedding any tears while crying
Signs that a toddler may be seriously dehydrated:
• Sunken eyes
• Hands and feet that feel cold and look splotchy
• Lightheadedness, dizziness, or delirium
• Excessive sleepiness or fussiness
And \link{http://www.babycenter.com/refcap/toddler/toddlerills/11407.html\here} is the page that is from.