The signs you need to look for dehydration
are sunken and listless eyes, irritability, lack of tears when crying, doughy skin, decreased urine output and fatigue
Your child has mitochondrial disease
which can itself present with gastrointestinal disorders. Also you mentioned that your child is having distention and pain even on giving Pedialyte
through the G-button.
In such a situation, you must take your child to the ER.
Apart from the signs of dehydration mentioned above, it will be easy to monitor her BP in ER as BP can drop when dehydration sets in.
So taking her to the ER will be a good option.
Wishing your child a speedy recovery.