They asked for a snack instead
If the snack is the same food already on the plate, offer that piece from the plate. If it is a different food, decide before dinner whether snacks pause while the meal is out.
Decide before you are tired
A calm no, plus the food they already eat, is clearer than a second dinner made from the snack cupboard.
If banana is on the plate and they ask for banana, give them the banana from the plate. The plate stays the place food comes from.
If the snack is a different food
You can say the snack comes after, or at snack time, and point to the food that is already a yes. Then stop negotiating the menu.
Inventing a new plate from crackers and yogurt, when dinner is sitting there, teaches them to order a replacement.
Milk and yogurt
If the requested snack is yogurt or cheese and milk is an allergy, it stays off. Offer the safe food they already eat.
Do not use a forbidden snack as the rescue. An empty offer is safer than a risky one.
A sentence you can say
“Banana is on your plate.”