I have been going away on writing retreats twice a year since our girls were 3 and 5. Now they are 6 and 8 years old, and, as you can see, the older one has started to wonder. Trying to answer "why" is like trying to explain prayer. I did try to break it down for her as best I could: I said I need some time with only my writing, some space to clear my head. I was ready to go all philosophical, but that was enough for her. But of course I continued to ask the "why" as I enjoyed a few days retreat.
I go away because I require a stretch of quiet to sift through my journals, write new poems, revise, and collect poems into manuscripts. From the time I was pregnant with our oldest, I have loved "nesting"--I vividly recall carefully folding her newborn clothes and placing them in the new chest of drawers. I continue to love the domestic organizing, the schedule shuffling, the activities that occupy our family day to day. It's a gift to have this busy-ness. However, it's an equal gift to free up that space in my mind as I can when on retreat. Creating makes me whole. I can see my world and the space I am to hold in it. And I am able to build up enough material and set the direction for the next several months when writing returns to being a sliver of my time.
I can't say I go away just for this reason, but a nice effect is that our girls see my writing is important to me. I am grateful they see that I have a partner--their dad--who supports me and that we can all be a-okay when I'm away.