What it’s like to travel and especially dine with small children? Our is 1 and 3 and not exactly behaved in Japanese nor European terms - they make mess and break things in restaurants. Will we have hard time in Japan?
If you just let them behave as normal you will have exceptionally hard time. Unless you're are really doing your best to correct them and meet behavioral expectations, then you'll find people are very forgiving. But truly, you'll have to show that you're trying.