You're already on the right track. Trailers aren't inherently a problem. If they meet U.S. requirements, they're going to meet Canadian requirements (at least as far as temporary importation is concerned; it might be a different story if you were planning to import the trailer permanently). It's also quite possible that your trailer won't be searched, although it's possible that it might be. Assume it will be, in the plan-for-the-worst-and-hope-for-the-best vein of thought.
As far as things to watch for, as a monthly crosser of the border, here are my experiences:
- Food is an issue at times. Declare all food. You won't get in trouble having it and declaring it; you might have some food seized but this won't reflect negatively upon you because you declared it. Fresh produce and meat are the particular concerns. There are limits in how much dairy product you can bring into Canada, so don't bring piles of cheese and milk, but a normal working amount won't be a problem. Going U.S.-bound, citrus is forbidden, even if of U.S. origin, because of concerns it could have been in contact with infested citrus produce from other countries. In general, Canadian and U.S.-grown produce can cross the border freely (notwithstanding that citrus comment) although there can be temporary restrictions if an area is having infestations of pests. See this website to get current information on what food is allowed into Canada.
- Firearms are a specific concern, surprisingly, in both directions. If you plan to take firearms with you, make sure to research the rules in advance. Ensure that no trace of firearms remains, if you normally carry them - if Canadian officers find ammunition, for example, they're going to tear your trailer and towing vehicle apart looking for the guns that go with them. Again, just be honest.
- If you're going to be leaving anything in Canada (or in the U.S., for Canadian travelers doing the reverse of your plans), declare it. Modest amounts of goods are generally let in without hassle. I've even taken old computers across the border to give them to someone along our route, and while I've gotten brief curious inspections, I've never gotten any hassle.
- Ensure that any prescription medications are in bottles identifying them and confirming the prescription. In particular, Canada allows certain codeine-based medications over the counter that the U.S. requires be prescribed. Don't export those to the U.S. unless you have a prescription from a doctor in your possession confirming you need them.
- Both countries seem relaxed about having reasonable amounts of personal effects. Don't go crazy with what you take, but if what you have makes sense, you should be fine.
- Declare anything you purchased in the destination country when you return to your home country. Both countries allow you to import $800 of local currency of goods per person after an absence of 48 hours. Anything above this is subject to duty and tax, although if you are not over by much, oftentimes the taxes are waived. If you plan to import tobacco or alcohol, there are special rules that you may want to research in advance, but you can take a reasonable amount of that special Canadian rye whisky home with you if you want.
Regarding the car seats, for temporary importation, what's fine in the U.S. will be fine in Canada. Seat belts and appropriate car seats are required in every Canadian province and territory.
One other thing: the minimum third-party liability insurance limit requirement throughout Canada is $200,000 Cdn. Ensure your policy has limits at least this high by getting a Canadian Motor Vehicle Liability Card from your insurer, or carry a copy of your policy showing combined limits at least this high.
Have fun and safe travels!