The verb 'see' can take both an infinitive or "ing" form but with a slightly change in meaning. If we want to say that we saw the whole action from beginning to end, we usually use bare infinitive which is the case with your example and a similar one below. I saw him work in the field. If we want to say that only we saw part of the action, use -ing form I saw the consultant waiting in reception.
(I saw the consultant. He was waiting in reception.)