Hi Kim!
Bringing my son to Disney when he was 18 months and 2, I understand completely where you're coming from. Sometimes those table service reservations hit nap time, or, if later in the evening, "night night" time :-) Or, if they had too much sugar in their system beforehand, table service doesn't become the best option.
If you're on the Deluxe Dining Plan, you're entitled to 3 meals, each of which can be either table or quick service.
If you're on the Standard Dining Plan, it is at the discretion of the quick service restaurant to accept a table service credit. Table service-to-quick service is a credit downgrade, and some restaurants will honor downgrades. My recommendation is if this happens, and you go to a quick service restaurant, just ask them first before ordering to see if they are a restaurant that accommodates.
Good luck to a fun, magical, stress-free Disney vacation!