Like the previous posts, I'm not sure what you're referring to with regard to "childishness", exactly.
What you may be talking about is a feeling of finally being able to completely let down your boundaries, in a sense, the "wide-eyed, childish innocence" implicit in the child's state of completely "boundaryless" openness.
I think that many of us with extensive childhood trauma don't form legitimate emotional boundaries, but rather calculatedly detailed shells which we present to the world. They're, in other words, boundaries of intellectual construction, rather than visceral boundaries established via the lengthy process of emotionally engaging with others and the world around us, and which so build gradually, layer upon layer, with the "Emotional Intelligence" that develops, as a result.
Having withdrawn due to early trauma, instead, we are without the graduated, level-by-level system of established emotional boundaries--the result being an "all or nothing" view of letting people in, rather than its being a gradual process.
And so when confronted with someone who we feel finally safe around--a therapist--we completely drop this calculated shell, revealing our as yet undeveloped, childishly open, emotional selves--which remain stunted due to a lifelong pattern of avoidance of emotional engagement as threatening.
This is referred to as being an "adult child", in therapeutic/recovery terms...and results from just that...from being so injured/threatened at an early age, that we shut down, stop reaching out, as it becomes synonymous with pain. The result being this "all or nothing" set of boundaries, the "all" taking the form of being completely encased in our calculatedly developed, consciously designed "shell"....which is only rarely dropped, when we feel safe, to reveal the as yet unevolved, and so childish, inner nature beneath.
I'm not sure if that at all describes what your referring to, but I've experienced the same sense, in such situations. At the time, I was unaware of this as being inappropriate, as I thought of it only in terms of "being honest about who I am". Not having any frame of reference, I was unaware of how inappropriate this complete and utter absence of boundaries was, in an adult. Of course I didn't display this to the world. The world only got "the shell". But more than one therapist remarked to me "are you ALWAYS like this?..." in a kind of startled chagrin. Then a kind of shocked, "...HOW??!" of amazement.
Once I was told, "I can't help you unless you behave here as you do normally". So if this is what you're referring to, in other words, I understand completely.
The problem I've encountered is that I "shut down" so early, that in more than one case, the T simply said, in a kind of discouragement, while looking into my eyes, "...but you're so far behind. I don't think I can help you."
There's a "window", after all:if you still "have it together" to the point that you don't feel the need to reach out for the therapist's help, and so, to become open to it, you're not a good candidate for treatment....but if there's so much work to be done, that one doesn't know where to start, or to have hope of ever catching up...then you can be judged as not a good candidate, for simply practical reasons.
Every therapist is different, of course, as is every person. Each one has to gauge how the demands you will place on their resources will affect them, and so, their other clients, and life in general.
I'm curious as to how your therapist has reacted to this, in your personal experience: is this something that they've suggested is a problem, or a barrier to treatment?