Garden Grille!! We did every character dinner in WDW our last trip in Dec. and this was our favorite. Great character interaction and great food. My kids are 2 and 4 and we spent too much time in Epcot last time--way too much walking and too little to do. However, this is close to the entrance so if you just wanted to do a half day or evening you could come in for an early dinner, walk the loop and watch Illuminations. That is my plan for our trip in September. Also, the kids get a dirt sunday dessert with gummy worms---they loved it and DH and I were able to keep eating our meal peacefully.
I haven't eaten at Tappenyaki but from what I have heard it is like any Japanese Steakhouse, like Shogun, etc. While we love eating at these back home and they do generally keep the kids entertained--we can do that anywhere. You can only eat with Mickey, Pluto and Chip and Dale in WDW. Also, Japan is at the back of the loop, so if you don't plan carefully, you will end up rushing back there to get to dinner and it's a long walk.
Good luck and have a great time!!