Finding cheap Japanese food is difficult. There's a coffee shop named Cocum rated 5 out of 5 that serves what you're looking for, but it's for adults only.
[['Cocum', 'eatType', 'coffee shop'], ['Cocum', 'food', 'Japanese'], ['Cocum', 'priceRange', 'cheap'], ['Cocum', 'customer rating', '5 out of 5'], ['Cocum', 'familyFriendly', 'no']]