I think this is a 180 and a good step towards GAL, which means doing things you used to enjoy (like listening to jazz) and doing something just for yourself. I suspect it might remind you of how you used to be before the kids took up so much time and energy as well?
But I think you need to clarify with your old friend that going out to listen to jazz isn't going to cause a problem for anyone he may have waiting at home (just shoot him a msg on FB) and then you'll know. You're making some assumptions and it would be better to know before you go out.
So, if it's not a problem b/c he's either unattached or he reassures you it's not an issue and it's just catching up with an old friend, I think you should go because it will be great for your confidence, and independence.
As far as H watching the kids, my ex and I always gave each other the first right of refusal. Let him know you have plans (and be vague... I'm going to see an old friend... no one you'd know... etc) and you wanted to see if he wanted time with the kids first. If not, you'll get a sitter - either way, no big deal. Be casual, confident! And have fun!
