When splitting a saved order, it is possible to insert more quantity to the delivery orders than the initial amount on the parent, even though the following warning displays: Not every remaining order quantity is 0.
Users can save or submit the order with more quantity as there is no validation logic after the warning.
Veeva provides the ability to update child orders directly. The pricing of the order only happens on the parent order. If customers decide to edit child orders directly, then this is their responsibility to customize the overarching logic to maintain parent order with delivery orders in sync.
CRM Help Documentation: Order Delivery Date Splitting