I see. That's an interesting question.
I see two problems: first, you'll have a hard time proving damages. Slander and libel are designed to give persons a recourse when another distributes publicly information which is false which causes economic loss. Which means it's pretty much restricted to when a person starts a blog, or publishes, or posts notices, which damage that person's reputation. And the damages have to be shown to be damaging to the claimant's monetary interests. So if someone posts on facebook that I'm a jerk, that's not really worth pursuing. Even if they say that they are unhappy with my professional work done for them, they can do that too since that's their opinion. But if they start a blog or put a full page ad in the local paper that I'm a terrible lawyer, rip off my clients, and should be in jail, then the line has been crossed. So even if you prove that they lied about you you'll have a hard time proving what your monetary damages would be.
Secondly, the actions they took actually occurred in Switzerland. I don't know anything about Swiss law, so you'll have to inquire with an expert in that field. But in Canada a family law litigant can assemble whatever evidence that they want about the other party, and then the other party has to argue that the evidence is inadmissible, or irrelevant, unreliable, or should be given little weight by the court. That's what lawyers do most often, argue about the nature and quality of the other side's evidence.
So if you're bringing a case against them, there are a couple of things you could do. You could bring an action against them to have a charge of perjury laid. I don't know what happens in Switzerland but such a recourse would be very dicey in Canada.
As well, you'd have to show a few things and prove them:
1) That they lied, or were reckless of the truth when they swore it to be true
2) That the court treated their evidence with enough weight to be persuasive to the court, and
3) That the evidence by them led to the result. i.e., that but for their evidence you would have won.
The third point is going to be very difficult to show. I don't know how you could possibly convince a Canadian court that the Swiss court would have given you joint or sole custody if it weren't for the evidence of these two individuals. There may have been other evidence which the swiss court found persuasive, and even without the evidence of these two may have came to the same conclusion.
That's complicated, but that's the truth as I see it.
Last point is that even if you are successful that won't undo the order. You'll need to bring a court case in Switzerland to change the order made there. Canada courts won't have any jurisdiction over custody issues regarding these children.
I'm sorry that this isn't what you wanted to hear, obviously. But I'm not helping you if I only tell you what you want to hear or give you false hope. I'll await your question or comment, please reply if there's more to discuss. If I've answered you then I'd appreciate a positive service rating; please don't shoot the messenger.