I do...in the shower, so my kids wont see me, my daughter saw me once right after, and I told her I got soap in my eye, she (is 2) said Yea that hurt huh? I hugged her so hard, she was patting my back and said all better!
It's ok for your kids to see you cry. In fact, it's better for their emotional development if they do. It shows them what to do if they ever feel like crying. Child friendly language like "Mommy is overwhelmed right now," or "Mommy is feeling sad" teaches them empathy.