I wish I knew why... But when you only mentioned "child part of you" it already freaked me out a bit My T already knows that she should avoid words like "inner world" or "inner child" I am not a child anymore!
Thus, I also understand that there really might be different parts of one person and under some circumstances I still might behave like a child (or rather just childish) but when i only hear about it it's like "brrrrrr"(I do not know actually if in English you use "brrrr" to express that you are freezing or have chills). Thus, I also have a strong reaction to these simple words and I guess that we both could find a reasonable explanation for our behavior; however, it still wouldn't change much, right? One think is what we (or at least I) think and understand and another think is what we feel (if anything of course)...