Dice buonanotte a tutti.

Dice is the third-person singular present tense of dire, meaning to say or to tell.

So it can mean:

• he says
• she says
• it says

Depending on context, it can also translate as:

• he/she is saying
• he/she says goodnight

Italian present tense is often broader than English present tense.

Italian often leaves out subject pronouns when they are already clear from the verb form or from context.

So instead of saying:

• Lui dice buonanotte a tutti
• Lei dice buonanotte a tutti

Italian very naturally just says:

• Dice buonanotte a tutti

This is normal and very common.

Yes. Besides he/she says, dice can also be the formal you form, used with Lei.

So depending on context, Dice buonanotte a tutti could also mean:

• You say goodnight to everyone

But without context, many learners first read it as he/she says goodnight to everyone.

Because a tutti means to everyone.

With dire, the person receiving the words is often introduced by a:

• dire qualcosa a qualcuno = to say something to someone

So:

• Dice buonanotte a tutti = He/She says goodnight to everyone

If you removed a, the sentence would not work the same way.

Here tutti means everyone or all of them.

Literally, it is the plural form of all. In this sentence, it refers to a group of people:

• a tutti = to everyone / to everybody / to all

Tutti is the masculine plural form, but in Italian it is also the default plural for a mixed group or when the gender is not specified.

So:

• a tutti = to everyone when the group is mixed or unspecified
• a tutte = to all of them if the group is entirely female

Because buonanotte is a fixed expression meaning good night / goodnight.

In modern Italian, it is very commonly written as one word when used as the greeting:

• Buonanotte!

You may also see buona notte as two words in some contexts, but buonanotte as a single word is very common and natural.

Because here buonanotte is being treated like the actual greeting being said, much like:

• dice ciao
• dice grazie

So:

• dice buonanotte = says goodnight

Italian often uses greetings and set expressions without an article after dire.

You can also hear expressions like dire la buonanotte or dare la buonanotte, but the version without the article is completely normal when focusing on the words spoken.

They are very close in meaning.

• dire buonanotte a qualcuno = to say goodnight to someone
• dare la buonanotte a qualcuno = to bid someone goodnight / to say goodnight to someone

Both are used.
Dare la buonanotte can sound a little more like a set expression, while dire buonanotte feels very direct and transparent.

It can mean either one.

Italian present tense often covers both:

• He says goodnight to everyone
• He is saying goodnight to everyone

The exact English translation depends on context. If this is describing a general habit, says may fit better. If it is happening right now, is saying may fit better.

Not exactly.

Dice buonanotte a tutti describes someone saying goodnight to everyone.

If you want to directly say Goodnight, everyone!, you would normally say:

• Buonanotte a tutti!

So the sentence you have is a description, not the greeting itself.

Yes, Italian word order is flexible.

The most neutral version here is:

• Dice buonanotte a tutti

But you may also hear:

• Dice a tutti buonanotte

The original order is very natural and straightforward, especially for learners.