Sure, the impact from Bident of Thassa applies to Breeches, and it should assault this flip.
The rule that applies right here is rule 611.2c:
If a steady impact generated by the decision of a spell or potential modifies the traits or adjustments the controller of any objects, the set of objects it impacts is decided when that steady impact begins. After that time, the set will not change. (Be aware that this works otherwise than a steady impact from a static potential.) A steady impact generated by the decision of a spell or potential that does not modify the traits or change the controller of any objects modifies the foundations of the sport, so it will possibly have an effect on objects that weren’t affected when that steady impact started. If a single steady impact has components that modify the traits or adjustments the controller of any objects and different components that do not, the set of objects every half applies to is decided independently.
Bident of Thassa’s impact simply creates an attacking requirement; it does not modify the traits or change the controller of any objects. So, it modifies the foundations, and may apply to things that weren’t affected when the impact started, particularly making use of the requirement to Breeches.
For distinction, an impact that claims “Creatures your opponents management get -1/-1 till finish of flip” does modify traits, so it might not apply to any creature that enters the battlefield later.
