Remove certain elements in one list based on condition from another list

The code that you have right now works perfectly, but is also O(n * m) since removeIf iterates through the List for every Children. One way to improve would be to store every child's personId in a Set<String> and remove every Person from the List<Person> if their personId is contained in the Set:

Set<String> childIds = childrenList.stream()
                                   .map(Children::getPersonId)
                                   .collect(Collectors.toSet());

personList.removeIf(person -> childIds.contains(person.getPersonId()));

Just another way of doing same but without mutating the original list:

 Set<String> childIds = childrenList.stream()
                .map(Children::getPersonId)
                .collect(Collectors.toSet());

        personList = personList.stream().filter(person ->
                !childIds.contains(person.getPersonId())
        ).collect(Collectors.toList());

It should have a bit of increased space complexity but you could take advantage of parallelStream here.