How to select multiple elements that are children of given element?

You'll have to reference #mydiv twice...

#mydiv > pre, #mydiv > div

I removed the extraneous div element selector as the ID is specific enough.

As far as I know, there is no shorthand for selector grouping.

See "Selector Grouping".

Although, with LESS, it is possible in the "Nested Rules" section.

The only way to reference the id only once is to use it with the * selector:

#mydiv > * {

which will apply to all elements that are children of that div.

Depending on what styles you plan to apply this might be workable (I typically use this to zero-out margins, padding and border styles), but it's still probably best to do it the first way because it makes it easier to make exceptions/changes later down the road.