Niseko like the rest of Japan has a very low crime rate and is a safe place to visit with children. In recent years there have been a small number of reported petty crimes such as jackets going missing from late night bars. A very small number of unattended snowboards and skis have also been reported missing from the base of the ski field in the past.
If you are arrested in Japan, even for a minor offence, expect to spend 21 days in police custody while your case is investigated. Bail is seldom granted to foreigners. Japan has a zero tolerance policy towards drug crime and there are severe penalties for any drug offences.