The authors of Locke and Key's third season, Cuse and Averill, have stated that the last half of the series will concentrate more on the family and how they get through the difficulties that are yet to come their way.
Because we are so accustomed to discussing the forthcoming season, this is a rather unusual circumstance to find ourselves in. In this particular instance, though, we are attempting to predict what will take place in the following two seasons.
That is something that is really tough to forecast. In my opinion, all of the Locke children will most likely return for the fourth season. I also believe that they will bring back their mother for the fourth season.