"Play" should be in its base form when used with "used to." The correct infinitive form is "to play." The sentence talks about a general habit or preference of the subject (the father).
The phrase "used to" is typically followed by the base form of the verb, which expresses something the subject regularly did in the past but no longer does. In this case, "used to play" is used to indicate that the father used to engage in the activity of playing.
The structure of "used to + base form of the verb" is used to describe past habits or repeated actions. Therefore, "used to play" is the correct form, as it indicates the father's past habit of playing.
The correct phrasing is "used to play" because it appropriately indicates a past habit or action that the father practiced regularly.