Cons:
Biergarten is a buffet. I hate buffets with little kids. Too much toting of their plates and mine back and forth, I never feel like I get to sit and eat. This may be just me, but it is something to keep in mind.
On the other hand, Teppanyaki is a hot grill, inches from you. They seat the kids on the end of the grill, where it is less hot. We took my four and five year old and they did fine, but parents with particularly squirmy kids, or parents who are particularly nervous may not enjoy having their children's fingers that close to a hot grill. There is room for two on the end, so you will not be seated next to both your children.
(I like both restaurants, but both have their problems when traveling with kids).