I always suggest getting a park hopper ticket. The reason being is that some parks you will be able to spend tons of time at, and others you may do relatively quickly. Plus, we love to eat at different places, and this allows you to spend all day at the Magic Kingdom, but eat at Epcot if you want.
If you only have 5 days, I would not go to a water park. They are great, but honestly, the parks are better.
As for a meal plan, I would definitely get the basic one with a quick service, table service and snack each day. It will be plenty of food for you, and it will save you TONS of money. For instance, dinner alone may cost you $40 plus per person, but the meal plan for an adult is around $42 a day. It pays for itself at dinner, and isn't even including the $10+ plus and the $3+ snack! It also allows you to eat at restaurants that may have been too expensive for your family. Also, make sure that you make reservations for places to eat NOW. You may already be limited as some of the best restaurants are booked immediately as it is available! We have tons of favorites, so let me know if you need some recommendations!
As for tips, know that you can bring food/drinks in to the park, so if you don't do the meal plan, plan on doing that. Also, water is free at any station that has a faucet. The water does have a "flavor" to it, but when it is 90+ degrees outside, you really won't care! Make sure to FastPass all rides that have a line longer than 25 minutes, and since you are staying on property, take advantage of Extra Magic Hours. Believe it or not, if you show up at 8 AM on those days, you can have a large portion of the park finished before the rest of the people are even allowed to get in. (i.e. We did ALL of Animal Kingdom by 12 PM this year by getting there at 8!)
We own a Disney timeshare and go every year, so feel free to PM me if you have any other questions! Have fun!