Canadian affair/transat as a company still exist and they have a London office. However, they do not fly direct into Halifax.
You won't pay £1500 for the six of you; those days have sadly long since gone (those sorts of fares have not been seen since Freddie Laker's days). Also July is peak season. Bear in mind that children pay adult fare at around 12 years of age on some carriers.
Its just under 665 miles from Boston to Halifax and you may well encounter some problems in driving the car from the US into Canada.
Not all companies will allow cross border travel, and in fact, there may be different rules between different rental locations for the same company. You will be violating your renters agreement if you cross without their approval, and all kinds of very expensive problems will come up if you get a ticket or have an accident. In addition, U.S. Customs actually requires that the rental company has given their consent for the car to leave the country so it had better be in the rental agreement.Most auto insurance will cover you in both countries but you must confirm this with your provider.
You must always carry your usual insurance card but you may need to get an insurance card for the country you are visiting as well. Your carrier should provide this at no charge. If you take out the rental agencies insurance they should issue you a non-resident insurance card which must be kept in the rental vehicle. Given all this potential hassle I would avoid the car rental idea.
Make sure you understand how the mileage rates will work when converting between metric and U.S. standard measurements.
Keep in mind that rental agencies are never responsible for legal fees, fines or tickets. You may see additional penalties or fees if they end up being assessed against the vehicle.
You will also each need an ESTA for all travellers coming into the US and if you are not Canadian citizens also an ETa for Canada for each person travelling.
Given you have children as well I would look at direct flights to Halifax (think Swiss fly there) or alternatively flights to the nearest airport that someone like BA flies into in Canada (probably Montreal).