Unless you actually build a 'raised floor' and insulate, the only other way I know you can warm the floor is to install a Warm Floor System. (Radiant Heat System) They're very nice but if you're not using the space very much, it may not be worth the expense. BUT, it is worth checking into the operating costs.
No, padding doesn't insulate. And sealing the floor won't help either. As far as my take on padding, as an aside, get good padding. It's not that much more expensive and will feel good on a concrete floor. (If you go with a Warm Floor System, you'll most likely have to 'raise' the floor a bit but I dont' know how much. Contact the Rodney @ The Warm Floor Center in the Follow-up of this post. (Read his 'take' on this.) Warm Room Alternative. He's an expert!)