I think the answer talking about the Principle of Insufficient Reason is a very good one, but I'll give a "keyword" that I've found helpful thinking about this kind of thing. And that is: "probability distribution". "There are two outcomes, therefore it is 50/50" assumes that the outcomes follow a uniform distribution (and that is in fact the assumption of the Principle of Insufficient Reason). But there are other kinds of probability distribution, like the Gaussian distribution, a Poisson distribution, and anything you could name, under which different outcomes do not in fact have the same probability. Another answer suggested illustrating this with dice, and that could be one way: use dice, or even just a coin, to figure out the frequencies of different events. For example, flipping a coin 100 times you'll find you get roughly 50/50 odds of heads and tails. But flip 2 coins 100 times and you'll find that the outcomes "two heads" and "two tails" will be less frequent than "one hand and one tail". On the other hand if you distinguish the coins, then you end up with 4 possibilities (2 heads, 2 tails, A heads B tails, A tails B heads) and those are all equally likely. And that itself points out that the probability distribution the events follow itself depends on how you parcel out the probability space, how you define an "event" - are you conflating the head-tails possibilities or distinguishing the two? In the former case you'll get an event that's twice as likely as the others, and the latter you'll get two events that are both as likely as any other. You can also look at different experiments and name the different probability distributions that emerge (I don't remember if my multiple coin example would involve a normal distribution or a binomial one but you can look it up).
This notion of "probability distribution" can be a good way to put words to the notion that just because you have X possibilities, doesn't mean they all have to be equally likely... and that saying they are is in fact a very specific thing to say, something you'll only do under specific circumstances (either they actually are equally likely, or you have no information to say they aren't and thus follow the Principle of Insufficient Reason).