I think you need to think about where you might want to eat. When our kids were that small, especially our first trip, it was important to us to do character meals. It was fun for us to see the kids with the characters and it was efficient to eat and visit characters at the same time. With that said, I found buffets difficult and not relaxing because of the constant getting food for everyone and trying to not miss characters and get pictures and autograph books. I much preferred the character family style meals because then I could sit and relax more and enjoy the kids enjoying the characters.
Likewise, I didn’t always find counter service easy or quick especially in places where you have to go to different stations for the food resulting in multiple lines and getting tables, but not all QS are like that. My kids are not mac and cheese and nugget kids so for us the QS kids meals weren’t always a good fit but if you plan ahead you can find other QS meals.
With two kids ages 5, and 7, the dining plan often does save you money. I know there is a lot of talk of breaking even or coming close but with kids, if you do a lot of the character meals, I believe you do come out ahead with the TS plan.
For tips you can add up the cost of the meals found on the website and add whatever amount you might typically leave 18% or other. The tips will add up so you have to budget for that.
There are also a lot of non-character table service places that are fun for the kids. Also, consider whether your kids do well in restaurants. My kids were good in restaurants but I also picked places that I thought they would find entertaining but not always “kid” places. My kids liked Yak and Yeti in AK, and that is not geared toward the kids. You can also get advice on these boards as to which restaurants tend to have longer waits/slower service and keep that in mind when selecting places.