Ytdyklly

In my story, 12 year old Ruth has visions from another place and time which lead her to gather a group of kids for a quest. She is told there will be 18 kids, but she can only find 17 with the specific requirement. They travel together and, upon arriving, discover a stowaway. This boy knows he belongs with the group but he doesn’t tell anyone until well into the book (the reader also doesn’t know). Everyone else thinks he just came along to annoy them. The 18th child is an important character and his journey is pivotal to the story.

Why 18? Because it’s a Jewish-themed book and 18 is an important number in Judaism (it is related to the word for life and is often used for luck).

There is no specific task that requires 18 people and there is no need to highlight the number too much. It's there because it felt right to do so and, at this point, the characters are set. The number comes up a couple of times early in the story and, so far, not since. I'm looking for subtle ways to reference it.

In what ways can I evoke this symbolism and incorporate it into the story?

How difficult/dangerous is their quest? You can have the 18 travelers manage to escape from dangerous situations unhurt time and time again, and generally keep having miraculously good luck.
Similarly, if there's a situation where the 18 aren't all together (suppose a couple of them wander off sightseeing), that's when their good luck should fail - and only reappear when all 18 are together again.

If you're hoping to make the symbolism a little more obvious (for non-Jewish readers), you could have one of them joke that it's because they have a lucky number (similar to how you might joke about having bad luck on a Friday the 13th). Or, when there's only 17 of them, one can say "We need one more for good luck" - or something to that effect.

I see two parts to your question: signaling that 18 is significant, and signaling why it is significant. Assuming that you'll have Jewish readers too, don't skimp on the first part -- you want to give them something to notice and figure out, too.

Another answer addresses ways to show why 18 is significant. To show that 18 is significant, have it show up in various ways in the vicinity of the kids -- at a meal there's a candelabra with 18 candles; something happens at 18 past the hour; something costs 18 coins; a shelf contains 18 books; an office or hotel room is #18; etc. For an example (from TV, not writing) of this sort of thing, consider the TV show LOST, in which the numbers 4, 8, 15, 16, 23, and 24 were significant both together and separately, and some of those references you only notice after the fact, once you're looking for those numbers. You can slip 18s into your story in the same way. Make sure they always have something to do with the kids, and the astute reader will eventually notice that there are 18 kids, too. Avoid having 18 show up in other contexts.

You've said it yourself: 18 = life. It follows that had there only been 17 travellers, they would not have come home alive. Preferably every child, but particularly the stowaway, must have a crucial role to play. Otherwise, they're not necessary, right?

You could play more with the idea. Your 18 travellers might turn out to be somehow necessary for the survival of people around them, making them a "lucky charm" for those nearby. Again, however, all 18 would have to somehow be necessary.

To a Jewish reader, the link 18-life is very very obvious, so it becomes a question of how you employ it. It's not something that might be missed, but readers might wonder how you put that symbol to good use. (Non-Jewish readers would have to google it, I suppose.)