We JUST went to Great Wolf Lodge in Sandusky (where I'm guessing you'd go) and we LOVE it there. I haven't been to Kalahari, but a friend has done both, and she says that Great Wolf Lodge is great for younger kids, but as they get a little older, they kind of outgrow the slides there, which I could see, as they are not all that spectacular for older kids, but fantastic for younger ones! They do an animated show in the lobby throughout the day, and it's so cute for the kids.
It's a beautiful, lodge like hotel, restaurants in the hotel, a nice gift shop, a nice arcade, etc. And the waterpark area is WONDERFUL for families with younger kids. It's not too overwhelming, my 5 year old could go on all the slides this year alone, if that gives you an idea. There are 2 tube slides, a few body slides, a large water climbing/play area, a lazy river, a zero depth entry toddler pool with a little climbing and slide area, a nice, family friendly hot tub (and an area for adults only), and a snack bar in the water park. They also have a nice kids craft area, and other attractions for kids that go on throughout the day.
We really do love it there, and spent 2 nights there as well. We had my son, 5, and his 2 year old and 9 month old cousins there, and we all had a great time.
I'd say if your kids are old enough to really enjoy Kalahari and take on all the slides there, then they might be a little too old to really enjoy GWL. If they are young enough that they couldn't really take on all the slides at Kalahari, then they might just really love GWL.
PM me if you have any specific questions!
Either way, have a great trip!!