I dont but I want to! I have a three year old and a 1-1/2 year old and am having a hard time too.
Taking the kids for a walk when we get home works out ok. I put them in a stroller, and give them snacks and water, and off we go. Sometimes we walk to a playground and as the kids play, I do lunges or step ups at the playground. One playground is next to a field, so I can jog a little circle around it, and still keep them in close enough proximity that I can get to them quickly.
Then, when we get home, they are good and tired, ready for dinner and bed. I make something quick and easy, or have something cooking before we leave.
Defintely, though, my gym days and my marathon running days are over, at least for the next few years.