They will only get dessert if...
If you have a special dessert that they really like, and it is reward for good behaviour at the table, then they will pay more attention.
Your grandchildren are old enough to sit nicely now; it's different when they are toddlers. Children can be wiggly, but should not be disruptive to others having dinner. I wouldn't worry about how they hold their cutlery though, it can be suggested but not worth tears. It will come to them eventually.
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