With 2 toddlers your priority is not "good" food, it's practical food. Roppongi Hills mall probably has easy food court/fast food options.

Not so sure about "just show up" suggestion - Japan follows the rules very strictly and exceeding room occupancy may present a serious challenge. That being said hotel will definitely try everything possible to accommodate guest needs but you may face suite upsell charge for example.
I strongly recommend contacting hotel in advance or at least have have a backup plan in case "just show up" doesn't work out.