Los niños se meten en el saco de dormir cuando hace frío.

Meter by itself means “to put (something) into (something)” and normally takes a direct object:

• Meten los juguetes en la caja. = They put the toys in the box.

When the subject is putting themselves inside something, Spanish commonly uses the reflexive form meterse (en):

• Los niños se meten en el saco de dormir.
  Literally: The children put themselves into the sleeping bag.

So “se meten” = they get in / climb into (themselves), which is why the reflexive se is needed.

Both can translate as “go in / get in”, but:

• meterse en suggests more of a physical movement into a container or enclosed space, often one that’s a bit tight or protective:

  • meterse en el saco de dormir – get into the sleeping bag
  • meterse en la cama – get into bed

• entrar en is more general: “to enter” a place or space (room, house, car, etc.):

  • entrar en la casa – enter the house

You could say “entrar en el saco de dormir”, but “meterse en el saco de dormir” sounds more natural for a sleeping bag (you’re kind of stuffing yourself inside it).

Because the subject is plural:

• Los niños = they (3rd person plural)
• The present-tense form of meter(se) for ellos/ellas/ustedes is meten.

So:

• El niño se mete en el saco de dormir. – The child gets in the sleeping bag.
• Los niños se meten en el saco de dormir. – The children get in the sleeping bag.

A few points:

1. Singular with a group is common when each person does the same thing with their own item and context makes it obvious:

   • Los niños se lavan las manos. – The children wash their hands.
     (Each one washes their own hands, but Spanish uses las manos, singular phrase for each person.)

2. Likewise, “el saco de dormir” can mean “their (respective) sleeping bag” in context. It doesn’t necessarily mean there is just one sleeping bag.

3. If you want to be very explicit that each child has a separate bag, you can say:

   • Los niños se meten en sus sacos de dormir. – The children get into their sleeping bags.

The original sentence is natural and doesn’t sound wrong; it’s using a kind of generic singular.

Spanish often uses a definite article (el, la, los, las) instead of a possessive (mi, tu, su, etc.) when it’s clear whose thing it is, especially with:

• body parts:
  
  • Se lava las manos. – He washes his hands.
• clothes and things you’re putting on or getting into:
  
  • Se pone el abrigo. – She puts on her coat.
  • Se mete en el saco de dormir. – They get into their sleeping bag.

You can say “su saco de dormir”, and it’s correct, but the article “el” feels more neutral and is very common when the possessor is obvious (here: the children).

Yes, “saco de dormir” is widely understood across Latin America for “sleeping bag.”

Regionally, you might also hear:

• “bolsa de dormir” in some countries.
• Just “saco” when the context is clear (e.g., camping).

But “saco de dormir” is a standard, safe choice for Latin American Spanish.

Spanish uses hacer for many weather expressions:

• Hace frío. – It’s cold.
• Hace calor. – It’s hot.
• Hace viento. – It’s windy.

Using ser or estar changes the meaning:

• Es frío. – He/it is (a) cold (person/thing) (describing character or a general quality).
• Está frío. – He/it is cold (right now) (temporary state of a person or object).

To talk about the weather in general, you say “Hace frío”, so:

• …cuando hace frío. – …when it’s cold (weather-wise).

Yes. Both are correct:

• Los niños se meten en el saco de dormir cuando hace frío.
• Cuando hace frío, los niños se meten en el saco de dormir.

In writing, when the “cuando…” clause comes first, you usually add a comma after it. The meaning doesn’t change.

In Spanish, subject pronouns (yo, tú, él, ella, nosotros, ellos, etc.) are often omitted because the verb ending already shows who the subject is.

• Se meten already tells us it’s they (3rd person plural).
• Los niños is explicitly named, so adding ellos would be redundant.

You would only add ellos for emphasis or contrast:

• Los niños se meten en el saco de dormir, pero ellos no se quejan.
  The children get into the sleeping bag, but *they don’t complain.*

Yes, but the nuance changes slightly:

• Los niños entran en el saco de dormir.
  Grammatically correct, but less idiomatic; entrar is more for entering places (house, room, car).

• Los niños se ponen el saco de dormir.
  Sounds odd, because ponerse is used for clothes you wear, not something you climb into:

  • ponerse el abrigo, ponerse los zapatos – put on a coat, shoes.

The most natural choices are:

• Los niños se meten en el saco de dormir.
• Los niños se meten en sus sacos de dormir.

Both sound very normal in Latin American Spanish.