Couple of questions.
1. Age of children?
They will always be my children, but sadly they're not so young any more (28 and 25)
2. Are your tickets for 7 days already?
Yes - Our tickets are for 7 days
If your children are a little older, I would be more inclined to say yes. TL has a lot of great slides for older kids.
If you're already planning 7 park days, I wouldn't take a day away since you are paying for it. It would be a long day to do a water park and then go to one of the main parks later. Water Parks wipe you out pretty good.
Either way, don't make the decision until the day of and then add it. If the weather is too cold that week, they will probably close TL also.