Mian, understand what I'm saying is theoretical speculation, not my personal experience. I experienced some regression in my therapy, but it was never playful. It was painful and often coupled with some degree of dissociation. My T's response at those times was always to respond to the child state, but as an adult would speaking to a scared and upset child: his tone would change, he would speak simply and directly, in an effort to provide a sense of protection and containment, enough to allow me to regain stability.

But we shared a goal of working through in a way that would keep me contained, functional and stable in my real life. So regressions were handled as they happened, but the goal was to respond in such a way as to move through them, not allow them to flourish.

There are types of therapy that engage such states and work within them. It can be valid but I think tricky both to be successful and to not overly destabilize a client. If such regression is supportive of a healthy goal, and the T is capable of handling it, then whether it appears as playful or protective or whatever is fine. But if your willingness to regress is serving an unhealthy goal, then that probably needs some discussion with your T. I think my concern is that it seems like there's an aspect to regression with him that you enjoy, but is the consequence of it healthy for your goals? That's tough to know.