I was going to suggest the gym with creche which is what I do but if you dont have that can you swap an hour with friends to look after the kids for an hour while you exercise and then you have their kids while they exercise?

With DD1 I often put DD2 to sleep in her pram and go to the par, or just sit her up on a rug to watch - we play chasey, football, running races etc. That way you wear them out too