We just returned from Marriott's Harbour Lake, which is just about a mile and a half from Seaworld.
I agree with the other poster, that for such a long trip, having the space of a condo is really more comfortable. You could have a washer/dryer and full kitchen, plus a living room, so you'd have more space to relax.
I got our week through Interval International on a "getaway" and it was only $350 for the entire week. This time of year, it is normally $350 per night. If you have any good friends who own a timeshare affiliated with II, they can book these for you - they are hit or miss, and you can't get a refund (prepaid). But that's beside the point really, as if you're so close to Seaworld, you can call a taxi for about $5 each way, so you don't need to worry about driving. And there is a grocery store quite close by too, so you could take taxis there as well.
There is also a lot of condos closer to Disney parks, and you can take taxis as you need. I just would much prefer a condo for anything a week or longer.
Harbour Lake does have elevators, so being more than one story isn't an issue. If you mean a townhouse, then yes, I'd avoid them. But our condo was like an apartment. Also the pool had zero-entry and a nice play area, with splashing and such, so it would be nice for any child I think.
Good luck with your planning!