There is no need for me to tell my husband since I'm not paying anything for therapy now. And I don't see why I would want to tell him; I may be shirking my duty as a spouse when I don't let him know, but on the other hand, him knowing would make my therapy less effective so I don't really have any moral qualms about it. (I have been in therapy before, with him knowing, so I do know what I'm talking about.) There are no practical problems associated with this, as my sessions are in the mornings before my workday starts, two days a week, when he's at work. I don't need to tell lies about where I'm going since it wouldn't occur to him to ask - I guess outright lying would make it problematic.

I don't understand what is childish about this. Dishonest, yes, in an absolute sense, but in another sense, not particularly.