If I read it correctly, you would be "dropping off the kids" to use the pool? I do not believe Marriott's business model is to be a babysitter. What happens if something happens to the kids while they are unsupervised at the property?

Now... if you meant that you and the kids want to use the facilities, you could ask at the front desk... however, I am not sure why the hotel would want to give away their facilities when you do not provide that particular hotel with business.

And, I would consider it stealing to take advantages of services as a property you are not a guest of....