À la marée basse, les enfants ramassent des coquillages près des dunes.

À la marée basse means at low tide.

Here, à is used in a time expression, similar to at in English. French often uses à with set expressions of time or circumstances:

• à midi = at noon
• à ce moment-là = at that moment
• à marée basse / à la marée basse = at low tide

So à does not mean movement here. It introduces the situation or moment when the action happens.

In French, adjectives usually come after the noun, so:

• marée basse = low tide
• marée haute = high tide

That is the normal order for this expression. Also, marée basse is a very common fixed phrase.

Because marée is a feminine noun.

• une marée
• therefore: marée basse

The adjective must agree with the noun in gender and number. Since marée is feminine singular, basse is feminine singular too.

Les enfants can mean the children or children in general, depending on context.

Here it sounds like either:

• a specific group of children already understood from the situation, or
• a general scene: children collect seashells at low tide

French often uses the definite article more broadly than English does. English might say children, but French often says les enfants.

Compare:

• Les enfants aiment jouer. = Children like to play.
• Des enfants jouent dehors. = Some children are playing outside.

So les enfants is very natural here.

Ramassent is the 3rd person plural present tense of ramasser.

Conjugation:

• je ramasse
• tu ramasses
• il/elle ramasse
• nous ramassons
• vous ramassez
• ils/elles ramassent

Since the subject is les enfants = they, French uses ramassent.

In this sentence, it can describe:

• a habitual action: At low tide, children collect seashells
• or a scene happening in the present: The children are collecting seashells

French present tense often covers both ideas.

Des coquillages means some seashells or just seashells in an indefinite sense.

It does not refer to particular shells already identified. The children are picking up shells, but not specific ones known to the listener.

Compare:

• les coquillages = the seashells
• des coquillages = some seashells / seashells

So ramassent des coquillages is the natural way to say they are collecting shells in general.

No. They look the same, but they work differently.

In des coquillages:

• des is the indefinite plural article
• meaning some

In près des dunes:

• des = de + les
• because près de means near, and de contracts with les

So:

• près de les dunes becomes près des dunes

This des does not mean some. It means of the / near the as part of the contraction.

Because près is normally followed by de when it introduces what something is near.

• près de la mer = near the sea
• près du village = near the village
• près des dunes = near the dunes

So près on its own is usually incomplete unless the rest is already understood.

Yes. French allows that.

For example:

• Les enfants ramassent des coquillages près des dunes à la marée basse.

That is grammatically correct. Putting À la marée basse at the beginning gives it a bit more emphasis and sets the scene first.

The comma after it is also natural because it is an introductory phrase.

Yes, there is a liaison.

• les normally ends in a silent -s
• but before a vowel sound, that -s is pronounced like z

So:

• les enfants sounds roughly like lay-zahn-fahn

This liaison is required in careful, normal French:

• les enfants → lez enfants

Not exactly.

• un coquillage often means a seashell or shell found at the beach, and can also refer to shellfish in some contexts.
• une coquille is more general and can mean a shell, eggshell, or even a typo in publishing.

In a beach sentence like this, coquillages is the natural word for seashells that children collect.