My question is how would you enforce an hourly rate? Would you go by and tell people they have to leave after their time has expired...? I think charging hourly will be more trouble than it's worth. You either need to charge a flat rate or don't charge at all. Obviously, you're trying to make money, so I get the need for a fee, but, and this is my opinion, you need to make the cost minimal and only charge for kids using the toys, not for adults. Then have a coffee shop, like you mentioned, filled with snacks and goodies for moms and kids. Make everything healthy-ish and have packs of little crackers, cookies, etc. plus 100% juice and water. I honestly think that if you charge a minimal amount MOST moms will pay for the extras for coffee, etc. Plus, I know that if I'm only spending a little bit I am more apt to buy a treat for my kiddos or whatever. If your admission is less you will also have fewer people trying to sneak in snacks and drinks of their own. Of course you will have the moms that won't, but by and large, I'm betting you will get people in with just advertising such reasonable rates.
We have a place similar to this run by a local church. There is no admission fee and it is filled with mostly Little Tykes and Fisher Price climbing and sliding toys. They have a coffee bar and I've never seen a mom without a drink in that place. Most people feel like they should buy something since the play is free. We also have a Little Monkey Bizness (I know it's national, but don't know if you have them) which sounds like what you're doing except on a smaller scale, and it's very successful. Their snack bar is great and my kids will play there for hours! Look up their website just to get an idea, and of course, yours won't be that big to begin with, but it sounds similar.
I think this is a great idea and well worth it. I would go there for sure! i think the key is to appeal to the moms and not necessarily the kids, at least for marketing purposes...of course the kids need to like it there! ;) Oh and a final note about marketing, if you do get yourselves up and running, I highly recommend trying to contact Groupon and getting set up on that. I know a lot of people recently who have bought things just b/c they have a groupon and have never been there before. It's a great way to get your name out! Good luck, I hope it works for you!