Firstly well done for self-reflecting and questioning yourself about what's really going on for you - it would be too easy to get swept along in the fun of it.
And I would recommend that you view it as a signal that perhaps your marriage could do with a bit of TLC? In my work as a Couples Coach I can assure you that it's very normal at your stage in life (young kids, working etc) to find that conversations with your spouse tend to be all about logistics / kids / practicalities. And let's face it most of those are pretty (very!) dull. Long term relationships can easily fall into this pattern - most do and perhaps yours has?
AND it's not particularly healthy for the relationship. (I imagine your husband would love a chat setting the World to rights also and is equally feeling bogged down in the Calpol so improving this will benefit both of you). I really encourage couples to get back in touch with why they are together, what are their dreams and how to bring more fun and connection into the relationship. Let's face it - you didn't get together to just both work really hard, have kids and be exhausted. There was some other magic and attraction that brought you together. Rediscovering more of that fun and connection is all very possible but it takes being conscious and intentional about making it happen i.e. thinking about what you want, finding ways to make it happen and both taking action. (I offer free hints / tips on making relationships more fun and connected at www.facebook.com/thecouplescoach
In the meantime you might want to be a little bit careful of spending too much time with the work colleague. It's easy to feel attracted to someone else but you know you don't want it to go further, and it's not going to help you making your own partnership fabulous in the meantime.
Wishing you best of luck,