Learn Italian: I bambini viaggiano gratis in autobus. - Children travel for free on the bus.

When talking about traveling by a certain type of vehicle in a general way, Italian uses the preposition in without an article. So, "by bus" or "on the bus" is simply in autobus. You would only use sull'autobus (on the bus) if you were pointing out a specific bus, like "the number 60 bus."

Why is there an I before bambini if the English translation is just "Children"?

English drops the word "the" when making general statements about a group (like all children). Italian doesn't do this! You almost always need a definite article (the words for "the") when talking about categories in general, which is why it is I bambini.

Since bambini is plural, why doesn't gratis have a plural ending?

The word gratis (meaning "free of charge") is an invariable word. This means its spelling is frozen and never changes to match the gender or number of what it describes. It will always be gratis whether it applies to one child or many.

The base verb is viaggiare. Why is the "they" form spelled viaggiano and not "viaggano"?