You can move back to Canada with the kids, but there's a catch... Since they're US citizens, they can enter Canada without a visa (under NAFTA), for up to 180 days (tourism/visits). Past that, they'll require a visa. I am not sure if they have passports already because they need them, for proof of citizenship, to enter Canada. The tricky part about obtaining passports (if they do not have them already), the process requires both parents to sign-off -- how do you convince the abusive partner to sign-off on the forms? Food for thought...
Here's info on their entry into Canada: http://travel.state.gov/content/passports/en/passports/under-16.html
Here's info on the passport application process (in case you need it): http://travel.state.gov/content/passports/en/passports/under-16.html
The other option is to obtain a certificate of citizenship for the kids from the government of Canada. In some situations, children born to a Canadian citizen automatically qualify to be Canadian citizens. In your case, the kids would have dual-citizenship; if they qualify for Canada too. Here's info on how to go about that process: http://www.cic.gc.ca/english/citizenship/proof-eligibility.asp
All the above is really important if you're considering moving back to Canada with the kids, temporarily or permanently. The advice offered by other posters will help in facilitating legal procedures like pressing charges or divorce (if you're legally married). In the event that you're not married, seeing a lawyer may be a good start - you want to get it right from the start. Reporting to the police is not a bad option, but if he's not held, then he MAY get really nasty after the episode. So I think professional legal advice is a good place to start, strategically speaking. My advice is on the assumption that there's physical abuse involved as well; I notice you specifically said, "emotionally abusive".
