Les enfants se mettent en rang devant le portail.

Because se mettre is a pronominal/reflexive verb here.

• mettre = to put
• se mettre = to put oneself, to get into, to start getting into a position/state

So:

• Les enfants mettent en rang... would sound incomplete or unnatural here.
• Les enfants se mettent en rang means The children line up / put themselves in a line.

In French, this idea is often expressed with se mettre rather than a simple verb like English to line up.

Se mettre en rang means to line up or to form a line.

Literally, it is something like:

• se mettre = to place oneself
• en rang = in a row / in line

So the children are arranging themselves into a line.

This expression is very common in school contexts.

Because the subject is Les enfants, which is third person plural: they.

The verb se mettre in the present tense is:

• je me mets
• tu te mets
• il/elle se met
• nous nous mettons
• vous vous mettez
• ils/elles se mettent

Since les enfants = they, you use se mettent.

The se is part of the verb se mettre. It refers back to the subject.

So:

• Les enfants = the children
• se mettent = put themselves / get themselves

In English, we usually do not say the children put themselves in line unless we want to be very literal. We normally just say the children line up. French often keeps the reflexive structure where English does not.

En rang is the normal fixed expression for in a line / lined up.

• se mettre en rang = to line up

Dans un rang would usually not be used here. It sounds like being inside a row in a more literal or unusual sense.

French often uses en in set expressions describing arrangement, state, or manner:

• en ligne = in a line / online, depending on context
• en cercle = in a circle
• en silence = in silence

So en rang is the idiomatic choice.

Because devant means in front of.

• devant le portail = in front of the gate

This tells you where the children line up.

If you said au portail, that would mean something more like at the gate, which is possible in some contexts, but it does not specifically express the idea of being in front of it. Devant is more precise here.

Portail usually means a gate or a large entrance gate, often outside a school, house, courtyard, or property.

Porte usually means a door.

So:

• la porte = door
• le portail = gate

In this sentence, le portail suggests the children are lining up outside, probably near the school entrance.

Le portail means the gate, referring to a specific gate that the speaker and listener can identify from the situation.

French often uses the definite article where English also uses the:

• devant le portail = in front of the gate

If you said devant un portail, it would mean in front of a gate, without specifying which one.

Not completely. In natural speech:

• Les enfants sounds roughly like layz ahn-fahn
• se mettent sounds roughly like suh met
• en rang has a nasal vowel
• devant le portail sounds roughly like duh-vahn luh por-tie

A few useful pronunciation points:

• The -ent in mettent is not pronounced.
• There is a liaison in les enfants: the s in les sounds like z before enfants.
• The t in mettent is pronounced, but the final -ent ending is silent.

So mettent is pronounced the same as met in this case.

Yes. The French present tense can often correspond to several English forms depending on context:

• The children line up
• The children are lining up
• The children get into line

All of these can fit Les enfants se mettent en rang.

French present tense is more flexible than English in this way, so the exact translation depends on the situation.

Yes, it is completely normal:

• Les enfants = subject
• se mettent = verb
• en rang = how they position themselves
• devant le portail = where they do it

So the structure is:

Subject + reflexive verb + expression of position + place

French often keeps location details after the main action, just as English does.

Yes, you could. S'aligner also means to line up.

So:

• Les enfants se mettent en rang devant le portail.
• Les enfants s'alignent devant le portail.

Both are possible.

But there is a slight difference in feel:

• se mettre en rang is very common and often used in school or orderly situations
• s'aligner can sound a bit more formal, descriptive, or deliberate depending on context

For children at school, se mettre en rang is especially natural.