"Children like candies and treats."
あめ と おかし。 candies and treats only. just candies and treats.
あめ や おかし。 candies and treats etc. candies and treats and something.
You'd use と if you're giving an exhaustive list; you'd use や if you're just giving some examples from a larger list.
と implies that you are exhaustively listing everything. So it's not really appropriate.
In casual language is it possible to leave out the が before the すき? It feels like it flows better without it.
No, since 'ga' is already used to mark "candies and treats" as the subject. "Children" is the topic.
Japanese is like my mother language Chinese. We don't mention plurality unless it is absolutely needed. 子どもたち is not wrong, but would be required only in specific context. Here Children is generic and it is not likely that we need to say 子どもたち.