Om vi bliver på stranden eller går tilbage til havnen, afhænger af, om børnene stadig vil lege i sandet.

Yes. In both places, om introduces an indirect yes/no question, so it means whether.

• Om vi bliver på stranden eller går tilbage til havnen = Whether we stay at the beach or go back to the harbor
• om børnene stadig vil lege i sandet = whether the children still want to play in the sand

So both om clauses are about uncertainty or alternatives, not about time or movement.

Because om and hvis are not the same in Danish.

• om = whether
• hvis = if in a real condition

Here the sentence is not giving a condition like If it rains, we’ll go home. It is expressing something that depends on an open question: whether we stay or go back.

Compare:

• Jeg ved ikke, om han kommer. = I don’t know whether he is coming.
• Hvis han kommer, bliver jeg glad. = If he comes, I’ll be happy.

English often uses if in both cases, but Danish normally keeps the distinction clearer.

Because the whole first part is the subject of the sentence.

• Om vi bliver på stranden eller går tilbage til havnen = the thing that depends
• afhænger af ... = depends on ...

So structurally, the sentence is like:

• [Whether we stay at the beach or go back to the harbor] depends on [whether the children still want to play in the sand].

Danish allows an entire clause to act as the subject.

Because Danish does not need a dummy subject here. The first om clause already fills the subject position.

So:

• Om vi bliver ... afhænger af ... = natural
• literally: Whether we stay ... depends on ...

English sometimes prefers It depends on whether ..., but Danish can directly use the clause itself as the subject.

This is because of normal Danish main-clause word order.

In Danish main clauses, the finite verb usually comes in the second position. The first position can be taken by many things, including a whole clause.

Here:

1. First position: Om vi bliver på stranden eller går tilbage til havnen
2. Second position: afhænger

So the structure is perfectly normal Danish word order.

Because the same subject, vi, is shared by both verbs.

So:

• vi bliver på stranden eller går tilbage til havnen

means:

• we stay at the beach or go back to the harbor

Danish often omits the repeated subject when two verbs are coordinated and clearly belong to the same subject.

You could repeat it, but it would usually sound heavier:

• Om vi bliver på stranden eller vi går tilbage til havnen ...

That is much less natural here.

Blive can mean different things depending on context. Two very common meanings are:

• become
• remain / stay

Here it means stay / remain:

• bliver på stranden = stay at the beach

This is very common Danish usage.
Examples:

• Jeg bliver hjemme. = I’m staying home.
• Bliv her! = Stay here!

So this is not about becoming something.

These are normal Danish preposition choices, and they do not always match English one-for-one.

• på stranden = at/on the beach
  Danish usually says på with strand
• til havnen = to the harbor
  til is used for movement toward a destination
• i sandet = in the sand
  children play in the sand, so i is the natural choice

So the prepositions are idiomatic and should mostly be learned together with the noun or expression.

Because Danish often marks definiteness by adding it to the end of the noun.

• strand = beach

• stranden = the beach

• havn = harbor

• havnen = the harbor

• børn = children
• børnene = the children

This is one of the most important features of Danish nouns.

A small extra note:

• barn = child
• plural: børn = children
• definite plural: børnene = the children

So børn is irregular, just like English child / children.

Because this is a subordinate clause, and Danish word order changes in subordinate clauses.

In a main clause, you would normally say:

• Børnene vil stadig lege i sandet.

But after om, Danish uses subordinate-clause word order, where adverbs like stadig, ikke, nok, and måske often come before the finite verb:

• om børnene stadig vil lege i sandet

So this is a very useful pattern to notice:

• main clause: han kommer ikke
• subordinate clause: om han ikke kommer

Here it most naturally means want to.

So:

• børnene stadig vil lege i sandet
  = the children still want to play in the sand

The verb ville can sometimes point toward the future, but in sentences like this, especially with people and actions, it often expresses willingness, desire, or intention.

Because at afhænge af is a fixed verb expression meaning to depend on.

So you should learn it as a unit:

• noget afhænger af noget = something depends on something

Examples:

• Det afhænger af vejret. = It depends on the weather.
• Det afhænger af, om han kommer. = It depends on whether he comes.

So the af is not optional here.

They separate the subordinate clauses from the rest of the sentence.

• The first comma comes after the opening om clause:
  • Om vi bliver på stranden eller går tilbage til havnen, ...
• The second comma introduces another om clause after afhænger af:
  • afhænger af, om børnene stadig vil lege i sandet

In other words, the commas help show the clause structure clearly.